The drain cleaners should be able to cut the clean out off for a price or pull the toilet.
I would not add a clean out that goes up stream.
I would call a drain cleaner that will give a free video check.
The rubber couplings that are out there will last along time and that is all there is unless you want to pour a lead joint.
If that was my house I would just up grade the dain and never think of it again.
I think now is the time to do it when the city will let you do it at a lower price.
One of the big Rooter Companies will do a free video check of your main drain if you call and ask. It is company policy to do a free video check.