All sounds great, except you do miss a couple of pretty important points.
1. If a site passes the core web vitals test, that's the limit of potential ranking improvements as far as Google is concerned. So, if you really do have a faster site in terms of your scores than other Invision sites, since a stock build on reasonable hosting will also pass those tests, you've no advantage.
2. A higher score on one of those speed tests may only equate to <100ms of real world difference to a user. Which they won't notice.
So, on that basis, is it worth the time, effort and/or cost, or would spending that on things that do make a real difference be better? It's up to you to decide if course, but I know what I'd rather do.
And in the meantime, Invision can continue to modernise the platform and tidy up the js and CSS as they've said they will.