You can roll the firmware back under Organization/Firmware Upgrades (as long as it was not too long ago). There is a curly arrow on the right you can click on.
Personally - I've given up trying to use 80Mhz wide channels - on both Cisco Meraki and Cisco Enterprise. Trying to get such a large block with no RF issues can be challenging. I just use "Auto" now, and the system allocates as much bandwidth as it thinks it can get to work.
You mention old clients - you only need just one client to try and connect using 802.11b and the through for everyone sinks. Typically I made the minimum connect rate 12Mb/s to forcibly prevent these clients (I would rather have better performance).
25.11 is a solid firmware version. I don't remember the stats for this version any more, but globally is by far the most used version of code for the access points.
I should also point out that the MR5x series is a 4x4 MIMO radio. Simply put, it is a higher spec radio.
ps. There is no MR34. Perhaps you have an MR33?