Flash partitioning and RAM size are hardcoded in the DTS, the image recipes and tplinksafeloader, all of these need adapting - forever.
The bootloader may- or may not need patching/ rebuilding, impossible to say from the outside.
I'd be with you, if we were talking about mt7621a and 'only' replacing the flash with a 16 MB chip - but mt7620a and the RAM are beyond what I'd be willing to do.