Thanks @Pulver ! That was a great help! the BE805 does boot using the mt7988a-rfb image. Just tweaked it a bit and provided the spi0 layout and voila! OpenWrt is running on Archer BE805 as one can see below:
It is the beginning, I guess. I did a nanddump of the mtd partitions (cat worked fine for me) now I do not know how to get the calibration data and other data segments on the mtd device. There are 6 partitions:
root@OpenWrt:~# cat /proc/mtd
dev: size erasesize name
mtd0: 00200000 00020000 "boot"
mtd1: 00100000 00020000 "u-boot-env"
mtd2: 03200000 00020000 "ubi0"
mtd3: 03200000 00020000 "ubi1"
mtd4: 00800000 00020000 "userconfig"
mtd5: 00800000 00020000 "tp_data"
The above layout was release via the GPL code via the TP-Link! and it is consistent with the stock firmware logs:
[ 1.557071] 0x000000000000-0x000000200000 : "boot"
[ 1.606943] 0x000000200000-0x000000300000 : "u-boot-env"
[ 1.646934] 0x000000300000-0x000003500000 : "ubi0"
[ 1.726935] 0x000003500000-0x000006700000 : "ubi1"
[ 1.806937] 0x000006700000-0x000006f00000 : "userconfig"
[ 1.886945] 0x000006f00000-0x000007700000 : "tp_data"