Can I Get The Best of Both Proxy Worlds?

velocity is your best bet, you could also take a look at infrared iirc

"Other implementations This is, naturally, not an exhaustive list. Alternative implementations of the Minecraft protocol may or may not work. We encourage you to experiment and to contribute back with your results."

I might be dumb, but what about
https://github.com/ViaVersion/ViaProxy

Only downside is that you'll need to learn java as you mentioned at the end, yh
Gate is a good idea tho (modified), if you can implement the protocol by yourself

Haven't used it myself, but seems more like a 1-to-M proxy (one server, multiple clients), safe for the Players GUI section, that 100% is 1-to-1
Maybe you could take some code from it, slap it into velocity and get something somewhat usable

Or have an awful setup like (I'm assuming you have both legacy backends and clients)

backend <- ViaProxy <- Velocity <- ViaProxy 

I just mentioned that ViaProxy seems like a 1-to-M rather than a 1-to-1

At least from what it says on the readme, you could ask in their discord for more info

The only part that looks like 1-1 is the players gui, the rest don’t

I took a lil bit of time to look at the config, if you use it as a server owner (not as a player) it works as a 1-M (1 backend, multiple clients)

https://github.com/ViaVersion/ViaProxy/blob/main/src/main/java/net/raphimc/viaproxy/protocoltranslator/viaproxy/ViaProxyConfig.java