Could you add simplified recipes from the other Yuoki train mods?
YIR - Yuoki-Industries-Railroads - https://mods.factorio.com/mod/z_yira_yuokirails
YIR - Uranium Power Trains - - https://mods.factorio.com/mod/z_yira_UP
YIR - Americans - - https://mods.factorio.com/mod/z_yira_american
Maybe when you're getting to the tech tree part.

Additionally, could you remove the weird speed cap from some wagons? I can't use modded fuels or the Zephyr's full potential since some wagons cap at 259km/h

I'll take a look in my spare time. Thanks for suggesting.

I also wanted this but noticed you never did find the time to look into it, so I decided to fork the mod and implement the other Yuoki mods as well. I hope you don't mind. You are credited. If you want me to take it down and share the code with you instead so you can update this one, do say https://mods.factorio.com/mod/yi-railway-simplified-forked

Or, something I've just seen is you can invite users to collab on mods, lets invited users edit and publish releases. If that is something you wish to do, I can deprecate my fork and update yours for you.