Ok I 2 bathrooms washer and dryer and a kitchen lol. My master bath and washer and dryer are closest to septic and have never had issues the kitchen has backed up once . Now I have a shower sink and toilet that back up all the time. I bought a new toilet had camera run down main line snaked the main one from clean out to septic. I'm going to try to explain the set up the toilet and sink run to shower ( which has some self venting thing ) it's a 2 ' -3' span from toilet to shower and from sink to shower. From the shower it goes maybe 5 ' turns them 15' and makes sharp turn to head to septic 4" lines all the way. At that last turn there is a y connector one end of the y is the clean out which lays flat not up! Now if everything backs up I simply go out open clean out all water and everything else (nasty) comes pouring out under my deck (sick) if you leave that open it drains and drains and works awesome if you close it everything backs up . But I want to be clear from that point down pipe to septic close to 100' there is no blockage it's been camerad. The biggest problem on top of this is there is NO WAy to get to pipes there is no access to crawl space . This happened once last winter and then has been going on for 3 weeks now . We also had septic cleaned a few months ago .
If you open the cleanout and waste from the house comes out there, then the blockage is downstream of that cleanout. As most plumbers will be happy to tell you, waste flows downhill, so follow that line downhill until you can identify the problem. I don't think there's much that someone on the internet can do to better narrow that down compared to a plumber that is looking at the pipes with a camera. If one plumber can't find it, it's probably time to get a different plumber.