MultiAVCHD works well for me:

https://www.videohelp.com/software/multiAVCHD

It works best if the source media is already packed up into separate BDMV folders - you then drag and drop them into the media list, and ‘merge’ to create a combined playlist.

It isn’t quite seamless (which shouldn’t matter in your case anyway), but it won’t re-encode unless you want it to.

Let me know if you need any more pointers on this - I’ve been burning discs like this for years, but now that I’m using 4K more I’m forced to used much clunkier player/media server playlists, which is a shame.