First, regarding homage.
If you decide to build a homage, you must select the model you want to build.
For example, as already mentioned, in the 44mm case there were only automatic movements.
Further - if you just want to build something good to your taste from the details that you find - you have no restrictions, many are not even on the homage forum, but on the forum, they do this, and by adding some of their individual parts, they even manage to sell .
If you want to make something based on the ETA6497 - go ahead, look for parts, read, assemble them, build, it's a long and interesting path for some.
As for the WR100M, Getat used to give a guarantee for its homages 100m, but now, it seems, only with a solid caseback, even if it's a radiomir. He did and you can, a matter of practice and time. And yes, having an ETA inside won't prevent them from keeping the WR, it just increases the risk in case of an error.
And finally: even bettarini parnis cases look different, cheaper, simpler. One good rep noob caseset will take you to another level of aesthetics. The same goes for lumes, fonts, everything. Parnis caseset costs $30, treat it like it is and wait for it.
When parnis appeared, how many copies were broken on the gen forums, that this is the same, but cheaper. Today, we look at 250 replicas, and we understand that they are far from gen, but a light mile from parnis.
As for "how to make your watch waterproof"? Buy a few kits, read and learn. Everyone will pass this way on their own. The path of trial and error.
Good luck to you.