debug: [console=ttyO2,115200n8 root=/dev/mmcblk0p1 rootfstype=ext4 rootwait coherent_pool=1M splash video=HDMI-A-1:1024x600MR@60e fbcon=rotate:3 usbcore.authorized_default=0 fsck.mode=auto fsck.repair=yes] …
For 6.x kernels you need to use ttyS and not ttyO (4.x kernels) as @RobertCNelson said.
I would imagine you can still use the same dtb file. There might be more drivier options in newer kernels that require extra dts file entries. The symlink option is possibly one such example, so there could be others.
If the board is similar enough I would try booting the standard BBB image, with your u-boot which you know works,
As you are getting no kernel output it is hard to say exactly where in the kernel the issue is.
Also don’t know if you have seen it, but there is a problem on the 6.x kernels with some eMMC wearing out and bricking very quickly. https://forum.beagleboard.org/t/possible-emmc-firmware-bug-or-hw-issue-recent-seeed-studio-bbbs-with-6-1-x-kernels/36648?u=benedict.hewson