I'm finding that the most needed hypothetical access panels would be in the glare shield, for access to the pilot (aft) side of the sub-panel from above. Will never happen of course - windscreen is in the way of any useful access from that angle.
I have access from below (by assuming the most painful sustained body position ever devised) and through the giant holes for the EFIS screens, so no real room for complaint.
The space between firewall and sub-panel on my build contains the FW ground block forest-of-tabs, the Dynon engine monitor module, the MAP sensor, SDS CPi2 controller boards and a harness connecting some left and right side avionics. Not much anticipated need to reach back there for routine stuff. The pilot side of the sub panel is easy access through the instrument holes except for the basket-of-snakes cabling coming out of the Advanced Control Module with some over-generous (IMO) service loop excess. The cables themselves really fend off any attempt to get to the Dsub retaining thumbscrews!
The most difficult access in the whole panel is typically trying to work behind the AeroSport 310 center stack with the carbon fiber tunnel covers installed. These side covers also house the alternate intake air and the parking brake Bowden cables, which I had the fortunate foresight to make separately removable from the panels.
Takes a contortionist with two wrists on each arm to hook up the GPS navigator tray connections.