Unfortunately it wasn't plain sailing to put rc2 on a Linksys MR8300 today, despite setting the kernsize variable to 500000 before starting. I had 22.03.4 on the second partition and flashed 23.05.0-rc2 (factory-image) from there.
After setting up a serial connection, I saw that the router reported
## Booting kernel from FIT Image at 84000000 ...
Bad FIT kernel image format!
ERROR: can't get kernel image!
Of course, with run bootpart2 I could still boot from my old OpenWrt.
In the end, I loaded 23.05.0-rc2 with tftp (you have to set a PC as a tftp-server, the address the router is looking for can be seen by issuing printenv from an U-Boot prompt), see the instructions here, booted that image, then proceeded to download an image on the router (i.e. in OpenWrt) and flashed it with sysupgrade.
Then I had a functional router again 
Despite having the serial dongle + wires around the house for a long time, it was the first time I used it, everything seems OK.