There's something I can say -- it very likely won't help your present situation, but you did say "upwards and upwards", and not just "upwards", so it sounds like you're asking a question about the structure that would need to be encountered if you did find a way to push the limits.

I would expect that the Unseen are always juggling a bunch of potential plans, adjusting their choices of threads of plan to be materially realized according to various competing priorities and various newly-coming-into-focus opportunities and blockers. However, I don't know when questions of lifespan and next-life arrangements might shake out "well in advance" during these adjustments, requiring a lot of upheaval and creating a lot of poorly-retied loose ends to switch over to a next-best option late in the game, or when they might instead be just as up in the air as everything else.

With respect to the priorities of the Unseen (which have some relation to the priorities of the soul), a single lifespan is subject both to what economists call "increasing returns" effects and to what they call "decreasing returns" effects. During a lifespan, the soul acquires an increasing amount of context and expertise for how it works to be that particular kind of person. So, later in that life, it can practice and learn skills and habits for being functional and effective as that kind of person, and the habits will be more refined, and more compounded on top of other habits, than the habits that would be available to be learned by a soul who hadn't lived that life as long. Likewise, the consequences of one's habits might become more interpretable, since one would have become more of a consistent kind of thing whose habits had more consistent kinds of consequences. Those would be major sources of increasing-returns effects. At the same time, after those habits and interpretations had progressed beyond a certain point of refinement, further improvements might be less valuable or meaningful outside the context of that one life, relative to what could be learned by switching over to another life. That irrelevance would be a major source of decreasing-returns effects. The Unseen's general target for how long a life should be would have a lot to do with the question of when the increasing-returns effects became slow enough that they were cancelled out by the decreasing-returns effects.

Also, one of the Unseen's other priorities would have to do with the problem of maintaining an overall tendency towards consistency of material cause and effect. (This is related to what Dion Fortune called the "initiation of the nadir".) If medical technology were more advanced, people might live longer, at least in some sense. (I read a lot of speculation in this and the other forum about how the senile-dementia epidemic reflects a deficit between the state of medical technology and how long it can keep people alive, versus how long it generally remains beneficial to souls to stay tethered to material consciousness. I have read elsewhere that a lot of our efforts to reduce age-related dementia, especially Alzheimer's disease, have been disrupted by remarkable amounts of scientific fraud. Presumably of course it wouldn't have cost too much for the Unseen to nudge people more susceptible to scientific fraud towards getting into that part of gerontology, relative to the future savings that would result from an epidemic of dementia causing souls to be more untethered from still-living bodies later on. For what it's worth, though, a lot of dementia seems to be a result of chronic systemic inflammation, similar to a more extended form of "chemo brain" or neurological insult from immune response to an intense virus infection.)

I am not sure what the relation of the tales of immortal (and mostly Eastern) alchemists to this hypothesis would be. Maybe, since they have come closer to meeting the Unseen's priorities halfway than materialist medicine has, the Unseen is motivated to come closer to meeting their desires for immortality halfway. More concretely, I wouldn't be too surprised if the crucial factor was that the alchemical procedures in question work partly as a microcosm of what the Unseen hopes to achieve with multiple lives. The vices that are cleared away by death and the virtues that are (at least speculatively) regenerated by reincarnation are, as I understand it, in Taoist alchemy loosely associated with the transition from Metal to Water. Metal corresponds to old age, planning, control, Scottian "legibility", rigid assumptions, and strategizing around remaining strength, and Water corresponds to... I guess dealing with (usually obscure and "illegible") elements of your character that would ultimately have to kill you if they weren't dealt with -- usually, by trialing elements of character to start over with, until you find some that you would have better reason to trust. (Perhaps Yin Water for dealing with things whose presence would kill you, and Yang Water for dealing with things whose absence would kill you.) That's sort of like what people call "formlessness" I guess? I don't know much about Taoist immortality concepts but I understand that quicksilver is often mentioned in that connection, which isn't so surprising since it looks like a way for something that's still a metal to possess certain attributes of water.

Correspondingly, to make it more systemically feasible and sustainable for someone's lifespan to extend "upwards and upwards", it might be necessary to fold in more and more of the ultimately-necessary functions of death into their ordinary ongoing life processes, until it wasn't clear where death ended and life began. (Well. I mean, fold in more of those functions, anyway. Stillbirth in response to before-conception genetic damage, and cellular apoptosis in response to within-lifetime genetic damage, are already such functions. And the immune response to cancer is a harder such function, for if apoptosis fails. I'm not sure what it means that people pursuing mystical contemplation are reputedly more susceptible to cancer; but maybe it is an expression of how they're in contact with a slightly more indiscriminate form of life, and so fail to have proportionately quite as much of the right aspects of death. And all of that is biological, when other necessary forms of renewal would be psychological and sociological and probably a few more -ologicals we haven't even invented yet.) Doing that right seems like a civilizational project on probably a greater scale than the whole history of, say, Taoist alchemy, and probably even greater than the whole history of Western medicine.

And if that were sufficiently taken care of, improvements in medical science and technology would perhaps also be necessary. (If they weren't already intrinsically bound up in the project. The same general principles ought to apply to figuring out what to do when some of your habits incorporate hard-to-replace dysfunctional assumptions and when some of your cells incorporate dysfunctional hard-to-replace genes. And, conversely, the reason our senile dementia research has failed seems to have a lot to do with failure of our scientific funding processes' "immune response" to researchers with "cancerously" fraudulent grant-acquisition strategies. Like, https://twitter.com/MicrobiomDigest/status/1848101502734504352 points to a rundown of Alzheimer's investigators tied to falsified data none of whom have had their labs taken away.)

I say this as someone with strong transhumanist leanings. An excellent materialist exposition of one of the core principles involved in the subject matter of your question can be found at https://gwern.net/Backstop .