toilet will not flush even after replacing the toilet twice

I bought this house 5 years ago and have always had problems with the half bath toilet. It was very very slow to flush if at all. Now it will not flush at all. I have replaced the toilet twice with new toilets, one round and one long. Both 1.28 tanks and not low flow. I presently called a plumber because the sink also got clogged. He cleared the clog but the toilet still had issues. He opened the wall to find the sink went into a cross fitting. The sink in from the left, another sink from upstairs in from the right, the toilet seems to be on the bottom and at 2 in the vent that only went up about 12 inches and was capped. So time running short that day we put it all back together and I later got a vent valve, with a box on the wall for later access, and installed a 36 in 1 1/2 inch vent pipe on the top of that cross. Mind you the cross fitting is about even with the sink fixture. Now the sink works great now but the toilet still won’t flush. We added a pipe to the tank so it will fill all the way up, still nothing. with the toilet off he inserted a water hose with a fitting and ran it for a long time, it drained to the septic fine (flow in the inspection hole outside was good). But once we reinstalled the toilet, same problem. Tank fills up and slowly does down and never really empties. If you put anything in it like toilet paper it won’t even do that. It’s as if it’s starved for air or something. He did notice the line going into the septic, the inspection hole is about 3 inches higher than the toilet line. He measured with the toilet off. This is from the center of the house to the back corner so about 60+ feet, not sure if that’s enough to warrant tearing into a slab foundation to replace plumbing.

that’s where we are and their only idea is to install a sump pump or tear the floor up and add a bunch of 45s to try and help it. Any ideas or even questions we having thought to ask yet. I and my plumbers are lost… on my third plumber now. Help…