This is not Ubuntu/Debian on x86 (where the distro team cares about kernel versions and that kernel and userland match) so there won’t be such ‘kernel update to 5.15+’ anytime soon. You’re currently running your RK3588 device with a forward ported vendor kernel that dates back to some 2.6 version.
Efforts to support RK3588 and its various IP blocks in mainline kernel have started recently but this will take a long time and eventually there will be some 6.x kernel supporting a number of subsystems. In half a decade maybe all IP blocks might be supported by upstream mainline kernel.
In the meantime you can hope on Rockchip forward porting their BSP kernel another time (in 2015 they were on 4.4 with RK3399, then on 4.19, currently on 5.10 and within a few years they will be on a more recent version that will be then Google’s required kernel version for the respective Android version Rockchip wants to support).