Easier question: what settings would be as close to yours as possible, in the hope the encode simply removes whatever issue is causing the blockiness

I know you use a 4.1 level (at least that’s what tsmuxer tells me when I load your files) - I just need a suitable Rate Factor to ensure that I encode with (assuming constant rate - though if I use max bixrate then that is apparently irrelevant, but would take longer as I understand. Then I simply need to set a max bitrate / file size?)

I know many people are no longer using blu ray, but I will happily share with anyone if I can get this to work.