Synthetics definitely need a rework for this path to be worth it. The first step looks great, but the second is ... really bad without major changes, and then you're locked out from doing anything like level 1 synthetic and level 1 genetic, which would be much more attractive.
For starters, something I've not seen mentioned - when I tried to play an all-synth empire I noticed a fixed 15% ethics divergence increase on the synths, which made them a huge pain to manage even beyond having to build them. (Which they do at least imply you won't have to on twitter) With that you're either going to be spending a ton of effort preventing them all going off and doing their own thing and being unhappy when you try to do what you originally made the empire for, or you're locked into individualist to make it desirable, an ethos pretty much entirely unrelated to the synth ascension. Not fun.
Secondly, the portraits are going to be terrible. As has been mentioned, the lack of distinguishment is already a pain with them being so similar to droids. Furthering this, it's impossible to tell if they're your synths or were leftover from another empire you invaded, or migrated, or any such thing, so you're confused why a planet is unhappy, then notice oh, these synths have pretty much the opposite ethos of everyone else. Oh, because they're not mine. So now I need to fight to get negative ethics divergence for them against their low happiness to make the planet productive. This expansion is only going to make it worse when your ascended synths, who are presumably going to be at least slightly different, still look identical, and if they have all the other synth properties, will also diverge in ethics, screwing you over further, as well as needing a massive infrastructure rework to power them. Yes, I can prepare more power, but I need to be doing other things at the same time, I can't dedicate all power to be spare for the soon-to-be-synths over, for instance, navy maintenance.
All for a specialist pop that anyone can get access to. If they haven't significantly reworked synths in general, particularly to not lose the original much more specific and useful benefits of your species, I probably won't try this path until there's a mod that allows one of one, one of another, or similar diversification. Which is disappointing, because thematically, it was my favourite idea.