thanks @pmeira, so I've loaded it with serial (according to your "recipe"), but I'm not big soldering master, so it was really messy soldering.. I don't know if I haven't overburnt something.. (but at least it seems everything is working as intended)
I've tried it also with "usual" option '2' - rewrite flash in bootloader, but it seems there's some problem with lzma compression of kernel - the error was like this one: How to replicate snapshot builds exactly? (LZMA ERROR 1)
I've tried to "look" on it with binwalk, but I just don't understand how can be dictionary bigger than the actual size of data (8MB vs 6MB):
$ binwalk openwrt-ramips-mt76x8-tplink_re305-v3-squashfs-factory.bin
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
9416 0x24C8 LZMA compressed data, properties: 0x6D, dictionary size: 8388608 bytes, uncompressed size: 6289259 bytes
2004840 0x1E9768 Squashfs filesystem, little endian, version 4.0, compression:xz, size: 3128791 bytes, 1175 inodes, blocksize: 262144 bytes, created: 2021-03-22 17:44:16
but when I compare this with "official" snapshot build of re305v1, result is very similar.. (& nobody has reported problems with it..
Could anybody test it with original oem upload? (but just be ready to "debug" with serial..) - in v1 it seems it was succesful: OpenWrt Support for TP-LINK RE305 v1 - #2 by petitess
I can upload my builds here, but I don't know what's "official" place where to upload it
btw. thank you @pmeira for your comments, actualy you we're right about macs negative increments (I've already updated my branch), but I don't think that positive increments would have any impact on functionality 
I have spotted in your changes, that you haven't changed size of target image size - actually you can use "bigger" size as mtd partitions were moved & they emptied more space for rootfs (it's more similar to re200 than to 305v1)