It looks a great project and I know these are a lot of fun to design and build. I currently have 3 Nixie clocks, one of which I published here, and the tubes for another 2 six digit clocks so I'll have something to do during the next lock down.
The tubes you have chosen (IN-1) tend, apparently, to have a relatively short lifetime because they are not doped with mercury. See: Nixie tubes (paragraph 4). If I had those, I'd probably also consider having a PIR sensor to switch the tubes off when the room is not occupied.
As for the fade in/out, I guess this could be achieved by varying the duty cycle on the cathodes of the two digits to be switched, over say a half second. You should be able to do that relatively easily, without having to deal with nuisance flicker, because your tubes are directly driven (not multiplexed).