This is nearly the perfect rapper for a retro console rom, if Mesen had support for Sega Genesis/Mega Drive it would be perfect.
As for current functionality, we already know Mesen can emulate more than just the NES, this is just NES. Mesen as it stands can emulate NES, SNES, GB, GBC, GBA, PCE, SMS, GG, WS, ColecoVision. Roms are still being made for all these and future support needs to be considered.
The other features just as aspect ratio, regions, filters, all the available input settings, shortcuts, a lot is missing. I know your goal was a torn down version of Mesen but this is too torn down. The publisher using this is gonna want all the features. Plus a possible multi-rom feature should be considered too. I say this mainly because publishers of homebrew games may not always like a one size fits all approach. You also mention Lowtek games using it to make their games playable on SteamDeck, their NesTek software allows you to change the icon for the .exe see if it’s possible to do that with with this.