causing my low back/tail bone/pelvic area to flare even in the most up right suggestions
Maybe it's the more upright position in combination with that saddle and the other geometry involved. What saddle is on your other bike? Maybe swap them.
When I have had to take significant time off from riding, my tail bone will sometimes have what seems like excruciating pain for a week or two every time I get on the bike. I just ignore it and ride anyway. After that week or two, everything is good .
If your pain feels like it's deep in the bones and joints, then I might just suspect it is that same pain I get. But if it's soreness of muscles or numbness, then that is definitely something else that needs finding out.