Instead of taking the two channels and averaging them out for mono, take the signal that is found in both channels only.
When you play a mono microgroove record with a stereo cartridge much of the noise and crackles are in one channel or another, not the centre, averaging out reduces the crackle, but if you could just select the middle of the sound field then a crackle in the left channel will be fully eliminated. More like playing with a mono cartridge.
I’m not sure I’m explaining this very well, sorry!
There’s a slight reduction in the volume, and theres a slight rounding of the top end, but the crackles at the start have just vanished. Jobs a good 'un!