Optiplex 9010 - New mobo to support Win11?

It's not worth replacing the board.  There are plenty of refurbished Optiplex 7060 and subsequent systems for sale at $200 or not much over that -- that model has an 8th gen CPU that Windows 11 does support.

And the 9010 is Dell proprietary.  All of their systems are.

I think updating board to a standard non-Dell microATX is a good option.  9010 power switch is proprietary Dell 5 pin but can be adapted to work w non-Dell board.

I was able to run Windows 11 on my 7010 by using Installation media that bypasses Win 11 install requirements.  My cpu is i7-3770.  It is not bad.  But it is certainly older and slower compared to current 12/13th gen intel cpu.  I would not spend $1000 on new Dell optiplex because those models you cannot upgrade later, whereas the old 9010 tower you can indefinitely because it uses standard mATX case and ATX psu.

More specifically, can't upgrade the MB on the newest Optiplex's.  Other than that, they have the widest array of options - PSU, CPU, RAM, GPU - that Optiplex's ever had.  Can actually be used for gaming with DELL OEM stuff.  The catch is PSU's are TFX and not ATX and the systems are mostly limited to Dell OEM, but do have some higher wattage options.  Another catch is they are likely out of budget.

@ejn63 's answer is the easiest unless you really want to go for it - installing micro ATX in the 9010 case.