Also old, and also had same issues with trying to hit switches in back left or right comfortably. I tried out a couple of 24-key X-Keys, one on each side, and have a control scheme going that I'm liking a lot. Feels like I can intuitively (at least for me) map any side instruments to them.
I have one on each side, mounted behind joystick and throttle, landscape style (6x4 buttons). Inside column of 4 buttons on each used by my thumbs are (from top to bottom) Win, Ctl, Alt, and a Mode Toggle button. Left win/ctl/alt for left X-Key, right win/ctl/alt for right X-Key, for my instrument modifiers. To the outside of the Mode Toggle button at bottom-inside corner on each side are left and right shift, for my action modifiers. Rest of the buttons are actionable. Mode toggle is separate set of DX buttons.. which I actually haven't had any use for mapping those yet since the Win/Alt/Clt + LShift/RShift scheme has worked so well for me so far.
For the Win/Ctl/Alt column of keys as my instrument modifiers... Holding the windows key on right X-Key i know i'm focused on the forward most instrument on right side (the one closest to the window), like the countermeasures control panel in A-10. Holding control on right X-Key (my back-most "instrument modifier"), I know i'm focusing on the back-most instrument on the right (the tough ones to get to). If I'm not holding down Win/Ctl/Alt, I know it's for the primary instrument directly to my side.
Then it's a matter of mapping the controls of that instrument (at least the ones I care about) to the 20-ish buttons left on the X-Key. Like the A-10 countermeasure panel, it's got the 4 buttons and 4 switches in 2 rows, those go on the first 2 rows of the X-Key pad. For more complicated panels, like the CDU, I use the Win/Ctl/Alt modifiers on the opposite X-Key to handle sub-layers.
On the bottom row, keys 2 and 3 to the outside of the Mode Toggle button, are left and right shift as my action modifiers. I don't use them for straight On/Off toggles. For dials or such though, holding down left shift will move a control CCW or down, holding right shift will it CW or up.. holding both shifts allow for 4th action, like toggling a switch cover.
I'm really liking it ever since I set it up this way. I haven't had a lack of keys available to map. It helps keep the focus on where the actual controls are in the plane, but much quicker than clicking on them with the mouse.
This would likely be a lot easier to understand if I through together a graphic. I can, if anyone is interested.