I agree this would be better, but I'm concerned about viability. Even with TH 40 and rolling at 46 from the ES boost, unless the complexity of Tweaks is super low the maximum TH TN that we can realistically do is 37; if we fail on a -9 swing we'll fail one in seven rolls, and because FP rerolls don't reroll the TN's roll we can't rely on them nearly as much as we would in Sealing. (It might well be the case that Tweaks have way lower complexity than making techniques from scratch, granted, in which case we could probably deal with failing on a -9; but we can't assume that.)
The problem is, if we commit to levelling TH and hacking ES and then it turns out that even TH 40 isn't enough to do that, we're kind of screwed without multiple unstagnations along the way. TH 40 from TH 10 is 1530 XP; we don't have to actually pay for most of that because of notes, but even if we manage to unstagnate our upcoming sealing barrier in the near future we'd be running into two combat barriers and another sealing one. We might not be able to level BoC quickly afterwards, if it turned out not to work.
Correspondingly, BoC 1 -> 30 is only 232 XP. It's less XP-efficient than levelling TH and tweaking ES, but it's faster, doesn't plunge us into stagnancy, and most importantly is guaranteed to work. I don't love the idea of essentially wasting a couple of hundred XP, but I like the idea of getting stuck unable to make substrate even less.
While I agree with the general point, I don't think it really applies here? Akatsuki already knows it's possible, because the Great Seal exists; and given that having a perfect image of the Great Seal stored in the Iron Nerve was only barely ruled enough material to unlock PS, I don't think knowing that we managed to do it would let them figure it out.
