I tried putting too many potato peels down my garbage disposer once and the result was a plug, then the line filled up, then any leak that the pipe had became a more serious issue since then the pipe was full of sewage. If there is a pinhole near the top of the line but the line isn't becoming full then it can go unnoticed. If the line then plugs and fills up, then you have a monster on your hands.
I guess there was just not much pressure or maybe the pipe was not that full. I had already replaced most of the line with plastic and thank god it had enough hangers because the drain got extra heavy when it filled up. I made the mistake of trying to use draino and then when I had to stand under the pipe with a big bin while removing the plug from the cleanout and put it back when the bin filled. It was pretty horrible especially since I had added draino. and here I was standing under it with a bin mostly full of water trying to screw the cleanout plug back in without getting draino in my eyes so i could go dump it and return to get the rest of it out of the pipe. I think the technical term is a conundrum
If you or sean could handle it you might be able to hack out a section and replace it with plastic , and add a cleanout. The more length you change the better. you can probably use a rubber boot with hose clamps to join the plastic to the iron to break the job into sections.
old iron is quieter, with plastic you hear the drain more. when I removed a lot of that old iron pipe it was caked with a clay like substance which had reduced the diameter to an inch or so. It's often easier to just cut any bad stuff out and replace it than tangle with unscrewing it. I saw some places where the threaded portion of the old iron pipe had weakened to the point it couldn't be used anyway. the plastic fittings and pipe are cheap.
I recently had to tangle with the bathtub drain and I replaced the super heavy old trap with a plastic one that can be easily taken apart. I still have some other sewer lines that I would like to take out , maybe re route outside. the toilet stack joins into that and that is super heavy so it pretty much has to be cut out in sections to remove it. Id put plastic pipe in the same place for the vent but it could go outside when it reaches the basement ceiling rather than being an eyesore.
maybe some want to preserve all the old piping but I think if you change it out to plastic in sections it makes it easier to cut out and replace any part. cutting the plastic pipe and gluing the fittings together is really easy. cutting iron pipe to specific lengths and threading it, then screwing it together is more difficult
If you put lots of easy to reach cleanouts you can clear any clogs without more disassembly.
if you have any holes in the line they may be allowing sewer fumes to get into the house. at minimum I'd wrap them up or something to close the pinholes to prevent the sewer gasses from coming up through them. something I found which was really mickey mouse and definitely temporary but worked was to clean the pipe put epoxy on the outside followed by wrapping it with aluminum duct sealing tape. later you can just saw the pipe and remove it , replace with plastic. I dont blame you for not wanting to play wiht the sewer pipe yourself. a plumber could replace that whole run in short order, he'd just cut the iron pipe out and replace it. I had a plumber who helped install my sewer outside have a quick look he figured he could re plumb the drains in a day and pointed out that I could remove a lot of that ugly old pipe from my basement and just box it in outside against the house.
I started running some pex water lines and I mean to replace more since a lot of that old copper is also very thin and just worn out. my plan is to replace all the water feed with pex. That will eliminate all the old shutoffs which are stiff and hard to close, with new ones. for now I at least installed a pressure regulator so it isnt; at street pressure. I dialed the pressure way down so I can;t have skin peeling showers anymore. no one complained so its running at only about 10 PSI but still sufficient for now, I just dont trust that old copper to hold a lot of pressure because I know it is thin and ready to have an issue so running at a low pressure is a temporary measure to hopefully prevent a burst.