I’ll see if I can dig out the program but it ran on XP via USB to the camera and ISTR there was no support for that when I moved to Win 7. Yep a couple of 555s would probably have been my choice back in the day. (a quick pulse to flip the mirror up then a couple of seconds delay to let things settle before starting the main exposure) but Arduino, Raspberry Pi, BBC microbit etc feeding a relay or optocoupler would do the job. I am surprised the cheap after market remote timers dont have a lock up facility but the ones I had a quick look at don’t seem to eg
https://www.amazon.co.uk/Neewer%C2%AE-Shutter-Release-Hasselblad-PowerShot/dp/B004FKYBJM/