Been there, done that so I know what you are going through. It’s not a pretty situation, is it? Your sewer pipes are shot so accept it and move on to solutions. If anyone tells you that clay pipes are still useable, why don’t people install them anymore?
One option is to completely replace the existing lines with new ABS or THICKWALL PVC. It all depends on what the codes are in your part of the country. I recommend ABS but that may be just because that’s what we use where I come from. Just don’t ever let anyone tell you thinwall PVC is OK. It’s not and the first time you must snake it out or it gets crushed by tree roots you will find out why. The good news is that you may be able to phase this in starting at one end or the other and do it in stages. Pay an engineer for their expertise before you do anything and follow their advice.
Keep in mind that if you choose to do it in stages, you may be able to do the work yourself. It’s really not too hard and is much more physical than mental.
Another option if it is available where you live is to line the pipes with a PE liner. Flexible black pipe that can be pulled through other pipes such as clay. I have done this on a property I owned and saved immense amounts of money. Again, talk to an engineer and see what is available.
You have my sympathies and I wish you the best of luck.