#### Second step: install snapshot or 23.xx
... Do not try to restore any previous 22.03 config file: the device now runs with DSA and need to be configured from scratch (meanwhile advanced users may know how to manually restore some settings). Backup your settings.....
The config file made from 22.03.5 is compatible with snapshots running DSA? Is version 22.03.5 for MR8300 running in DSA?
No. When a target moves to DSA, the general advice is to reconfigure from scratch. The network syntax is not the same. If you know how to deal with config files, you can try to manually recover some afterward (wifi for example).
No. Only snapshots are running with, and so 23.xx will.
Once OpenWRT will be running and working, if I want to sysupgrade OpenWRT without loosing config files.... do I have sysupgrade from Linksys partition (web) or directly from OpenWRT partition via shell/web?
My idea is to preserve stock firmware/partition in case of a bad flash or whatever.
You want to preserve Linksys firmware into a partition.
Save OpenWrt settings. Boot to Linksys fw, using luci-app-advanced-reboot or the manual trick with the power button. Than flash a factory image, the only the Linksys fw understands (no sysupgrade). After the reboot with OpenWrt, restore the settings. The whole process is equivalent to an upgrade, on the same OpenWrt partition.
I have kept a Linksys fw partition since I bought the device, but recently I put OpenWrt on both partitions. It works very well, I haven't experimented any issue. The dual boot allows to recover everytime. From one Openwrt partition, I can sysupgrade to the other, and settings are preserved. I'm using snapshots though. As far as I understood 22.03.5 is working in the same way.
Finally, since 22.03.3, OpenWrt can also flash a Linksys fw if you want to restore it back.