While I highly appreciate your posts about what you pulled off, and all the details are really useful, I think it is safe to say that fallible humans with limited resources won’t be able to replicate your solution anytime soon.
I call this situation alarming. Applications developed with Meteor won’t stand a chance SEO-wise in face of the new core web vitals, except in a few exceptional cases of those who, like you guys, with great effort and determination build a custom solution around what in fact should be provided in Meteor core.