We are having an issue with a couple of Mr-34 that were working fine before we upgraded to the latest firmware but now any clients who connect to these AP's have a really slow connection.
I would work directly with Meraki Support on this issue, there are some known issues with 24.12 that can impact MR32/34 APs and Support cold verify if you are hitting a known issue and suggest the best workaround. The 25.9 recommendation is a solid one, and 25.9 is also the stable release candidate, however also has a known issue that might impact your MR34 APs. Best to have Support review your full environment and suggest the best firmware to be on. You can also review all of the release notes on the Organization > Firmware Upgrades page in Dashboard.
If this was just a normal upgrade to the latest GA firmware, Support can roll you back. If you have a combination of APs other than MR34, Support can also pin the MR34s to a specific firmware version to avoid the issue, while all other APs could run the latest GA or SRC.
Augie - Did you ever get this resolved? We have tons of MR34's and have had several issues with Meraki's 25 and 26 firmware versions, so much so we are running 23.11 on all MR34's.