You push the slider all the way forward in order to screw the bolts in the back and viceversa to do the front ones.
This way you have some clearance to work with it.
It still can be tricky and take some time but it can be done.
The room inside the slider is little sparse, so best use an Allen bolt.
In real life cars the seat sliders are fixed with M8, on the simrig M6 is plenty because we don´t need crash protection.
So depending on the bolts used on your profile M6 ore M8 will work just fine.