I'd really love feedback and suggestions by the way! I have a fair amount of experience in maps but I've never just built an entire maps web app from scratch so I'm kind of stumbling through a lot of it, especially the web frontend stuff.
For a single solution to geocoding and autocompletion, there's Pelias: https://pelias.io/
OTP is actually really fantastic and it might be a better choice than valhalla's multimodal especially if it can handle GTFS realtime. Thank you for the reminder that it exists!!!
would love to have IPv6 support. It should be on default by now, if there aren't any issues hindering support.