Just a quick update, I got an image to boot and run:
[ 0.000000] Booting Linux on physical CPU 0x0
[ 0.000000] Linux version 5.10.26 (sn@DESKTOP-TPH49OG) (arm-openwrt-linux-muslgnueabi-gcc (OpenWrt GCC 8.4.0 r16313+21-851dadc257b7) 8.4.0, GNU ld (GNU Binutils) 2.34) #0 SMP Fri Mar 26 20:43:29 2021
[ 0.000000] CPU: ARMv7 Processor [512f04d0] revision 0 (ARMv7), cr=10c5787d
[ 0.000000] CPU: div instructions available: patching division code
[ 0.000000] CPU: PIPT / VIPT nonaliasing data cache, PIPT instruction cache
[ 0.000000] OF: fdt: Machine model: Netgear Nighthawk X4 R7500v2
After a few attempts configuring DSA (one of which made my device fail to finish booting - it's fragile to mistakes) i did get my AP only, 2 VLAN network up but it was not stable wrt to ipv4. I don't know why yet as my testing window closed when my family started asking for internet (I did not get a chance to disable eee as described above).
I can post a gist with a full dmesg, regulator_summary, plus some other system info (temperatures and cpuidle seem to be working) if you like. I'll see if i can test again tomorrow and try disabling eee to keep the network stable for longer.
Note I still get the build error I mention above. First time through after make clean/distclean no images are generated but after doing /usr/bin/make -j 1 IGNORE_ERRORS=m V=s immediatly after the build failure, images are generated (which i used for the test) but the same compile error still comes up.
I don't know why this happens yet, it looks like the command that generates qcom-ipq8064-g10.dtb silently fails.
HTH