Maybe it will be also good to look at the SteamOS distribution and what limits they are using.

According to the proton document [1], SteamOS also has an increased fileno hard limit. I haven't verified the actual value, but they probably just inherit it from Debian. And on SteamOS they really wouldn't need to go lower than on Debian, it's a gaming-only OS.

[1] https://github.com/ValveSoftware/Proton/blob/proton_3.7/PREREQS.md#fd-limit-requirements