I've recently (attempted to) upgrade my TS-209 to buster and the issue that I ran into is not the size of the kernel but the size of the initrd, which on my system has blown out from just under 4MB to over 6 (with xz compression). The main culprits seem to be the lvm binary (1.6M in stretch, 2.6M in buster) and the addition of libcrypto (2M) as a dependency of kmod and udevadm.
By pinning lvm to the stretch version I got the initrd to just below 5M, so it would fit into a combined RootFS1+RootFS2 partition. It looks like on the u-boot side I would just need to bump the initrd size from 0x3fffff to 0x4fffff in the cp command and the kernel command line, but is there a way to override the partition table the kernel uses? The TS-209 doesn't use a device tree.
Thanks, Karsten