Yeah I am not really experienced in high power circuitry, I only have experience with low voltage circuits and digital stuff. I have found a few relay modules for Arduino, but I am not sure whether they are meant for high current AC switching (they are marketed for switching mains voltage). I was trying to find a solution according to what you recommended about putting the timer behind the PSU rather than in front of it to turn the bulb off instantly with no delay or fading. I have a normal enlarger Timer (like this one: https://www.ebay.com/itm/1254797807...mNFIIYVkmROXLsXofgcEVVepU=|tkp:Bk9SR-C_6o2tYQ) made for switching mains voltage and every other timer I have found on the market is like that. The only one that works more like what you suggested that I have seen is a durst timer that is connected to the PSU (also durst) by a dedicated port on the PSU.
I have a Jobo/LPL 7700 with 12V 100W halogen bulb and when using it with the original Jobo PSU that puts out 12V AC and the timer connected between the PSU and wall socket I do indeed see a bit of delay/fading as the timer cuts power to the PSU. I have seen other people on youtube using a dichroic enlarger with the same setup and also having this fading of the bulb, but they do not seem to worry about it. With longer exposure times it probably doesn't matter anyway but with shorter times it should have an effect.
I have a Jobo/LPL 7700 with 12V 100W halogen bulb and when using it with the original Jobo PSU that puts out 12V AC and the timer connected between the PSU and wall socket I do indeed see a bit of delay/fading as the timer cuts power to the PSU. I have seen other people on youtube using a dichroic enlarger with the same setup and also having this fading of the bulb, but they do not seem to worry about it. With longer exposure times it probably doesn't matter anyway but with shorter times it should have an effect.