But it's blocking me for installation of some plugin like openvpn, because kernel isn't up to date.
Do you if it's possible to migration to an official version ? And if yes, how have i to migrate without loosing all settings (Vlan, Wifi Hot Point etc.)
For OpenVPN - there is very short window of time you can install kmod-s while repositories are at same version, in longer term you may need auc or luci-app-attendedsysupgrade to keep up with upgrades.
It will be stable with next release, 24.x or 25.y, device support is unlikely to be backported to kernel 5.15, like it is often done with devices differing just dozen DTS text lines from similar ones.
Config migration to stable - it is not 100% guaranteed, only major->major migrations are done. But usually works if you move to early -rc version then to x.y.0
device is so distinct from stable releases that nobody dared to port support bits to old kernel in stable release
snapshots move fast, you need to install openvpn kmod via firmware-selector or in few hours after upgrade
Migrating settings works (unless documented to not work) between major releases, but not snapshot to release, you may need to re-type from unzipped config backup
I think you are too early in beginnings to start with deep theoretical questions.
First install LuCI gui: https://openwrt.org/docs/guide-quick-start/developmentinstallation
Then install luci-app-attendedsysupgrade and first upgrade from there and withing hours after upgrade install openvpn. Will make your waiting for stable release less suffering.