Nice, been looking forward to an USUM version of this!
Here's a few suggestions that jump at me, feel free to consider or ignore them, I love and appreciate that this team builder exists either way ^o^
1. Not sure how how much sense it would make at this point, but I'd like a version (or as on/off option) of yet unrevealed designs (the new UBs, Necrozma form and the mythical is all I think?) to have "spoiler" icons/image instead, so I could send the link to friends who would be annoyed if I sent it to them with those visible :B
(possible easy alternative?: make "other" category not visible by default and put a spoiler warning next to its checkbox?)
2. It would be cool if the available PKMN icons were visually separated into categories (perhaps first big group=everything in the actual Aloladex, second group=postgame additions?/all those legends, third group=island scan?), the reason being that it's not immediately clear on whether you can realistically plan to...say include a random Sinnoh starter in your team before beating the game, or if it's just in the list because there is a way to get it in the game, even if it's absolutely post-game with a convoluted method (or can we use island scan mid-story? would be really dumb for gamefreak to not include those in the aloladex if that's the case).....or at least all those island scan non-aloladex exclusives should be in the "other" category if I'm not misunderstanding what it's supposed to be.
3. Minor subjective visual nitpick: I like that the border around the circles of the icons matches the secondary type, but feel like it's not clear enough that way. Firstly because it uses a darker colour (which matters when there's like 5 purple-ish types in the game), and secondly because it's so thin that some might not even notice this neat detail. My suggestion:
4. Not sure about implementation, but... we can pick what types are to be shown. However when I want limit the selection based on types, pretty much every time I want certain types (like ones I already have on the team) to not be shown. If I uncheck Dark, there is still all Dark PKMN showing that happen to have another type.