This is the cheapest interface that can work for you. It was used to flash ls1 pcms, so it is proved to work with larger messages.

https://www.scantool.net/obdlink-sx/

The only drawback, it doesn`t support x4 mode.
There is a free tool to load an updated firmware. The firmware file is available to look at also. I am not sure how it is compiled but upon a request you might be able to get a source code and make it x4 compatible.

Pcm can be flashed in x1 mode, so that shouldn`t bother you. It is just 4 times slower. With that chip size the write times will be acceptable and close to obd1 pcm write time.

The main difference between x1 and x4 is that in x4 mode the rounding of the signal is removed and it is true square type of transition and the timings are cut by 4.

The 12 bytes frame size limit is software limitations. When the PCM enters datablock mode the limitation is removed and you can send as long as you want.