I was wondering if there were really advantages to changing the kernel, I know keep it up to date is really important (4.9.131, 132, 133 etc) but 4.9 to 5.4, 5.10,5.15 ? Just asking cause I see on dd wrt, their kernel is still on 4.9 and upgrading the kernel seems to be not really important, maybe I'm wrong too, just asking?
Updating between 4.9.131 to .132 to .133 is not very important unless there is some security issue that is addressed between the versions. Updating from 4.9 to 5.4 to 5.15 means you might get additional features and fixes which are not "only" related to security issues. As for 4.9, it is (was) a LTS until January 2023, which means after that, it will not even get important (security) updates.
Bottomline: the whole point of flashing OpenWrt (or dd-wrt or.. anything non stock) is to have a working system which can be "easily" updated/upgraded because of security issues or when new features become available.
Specifically: Most of the supported wifi routers can't do "WPA3" according to the Vendor, but when flashed with OpenWrt, those devices CAN do WPA3 extending their life by a lot, even if they are limited on RAM/ROM. All OpenWrt devices which can be updated are automatically "guaranteed" to be secure and probably can be updated again whenever other security issue is found. Your OEM will likely not update their firmware "ever".
Driver development etc. happens upstream, so you need to keep pace with that. While stable OpenWrt typically does not use the most recent LTS kernel, you will notice the wireless drivers are backports from a more recent LTS. E.g. OpenWrt 22.03 uses the 5.10 kernel but the backports are from the 5.15 kernel. Likewise, master is moving to 5.15 but the wireless stack will be based on 6.1 (the newest LTS) soon.