I had only little problems to install gapps, but thanks to the instructions of @The montain I was able to to install gapps from opengapps.org (https://forum.xda-developers.com/redmi-note-9-pro/help/gapps-flashing-error-70-redmi-note-t4128649). So LR TWRP is stable enough. AFAIK it has more to o with the file system of android10.I don't think TWRP is stable enought to enable gapps installatiion.
I think that's the reason they are included.
One ROM builder answered to to me to the question if he would consider a version of the ROM that he maintains, that it would too much work and the reason why he includes the complete gapps pack in s ROM, is because "that's I how I use it".