Locating Loose Neutral

Working on my own 2-story house built in 1947 that has been, umm, "remodeled" by a house flipper right before I got it. There is a new 200A main panel, but the way it's wired, well, let's say, is still old school. There is just one breaker, 15A AFCI, that controls both upstairs bedrooms, upstairs bathroom with GFI, part of living room, and some other random parts of different rooms wired to it. I'm assuming it's likely how it was wired back in 1947 because other "extras" that have been added over the years are now on separate breakers.

Problem - one outlet has not been working ever since I got it 2 years ago - just now starting to troubleshoot it because it never gets used. Nothing works when plugged into that outlet. I actually measured the voltage on it and got 27v out of it. Did some googling and found out that it's likely to be a lose neutral "somewhere" or a bad backstabbed connection.

Started by poking around various devices on that breaker with a non-contact current tester and a multimeter. All devices actually work besides this 1 outlet. Some devices read 110v as they are supposed to. Some switches read 27v when measured with a multimeter, but the lights connected to them still work fine, most lights are LED. Still strange.

I found several switches that have been "backstabbed" instead of using side screws and thought I got it. I replaced those switches with brand new ones, since they were old anyway, but nothing changed. So the problem is somewhere I haven't checked yet. There are around 40 different devices on that circuit.

Question - what is the best way to locate the loose neutral? I can pull apart (and replace) all switches and outlets to make sure there are no wires that are physically lose (or have been backstabbed by the house flipper). I suppose I can pull all light fixtures and check those too. Doubt that the lose wire is inside the panel, possible, but doubt it because of 110v on *some* devices.

All boxes that contain devices have no wire nuts - they are done the old school way - soldered together and wrapped with tape. Am I supposed to take each of those bundles apart and redo them? I'm afraid of running out of wire length and having to run new wire. It's a 2-story house and running new wiring would not be a fun task. I just did a full rewire job on my other 1-story house of same vintage next door and I'm not looking forward to such a task in a 2-story building.

Thoughts on where to start? Ideas on why some devices are 110v and some are 27v if they are on the same breaker (and probably should be sharing the same neutral)? Any other words of wisdom? I've called a good electrician that I've worked with before and he said that he would do the same - check all devices one by one - which can cost quite a bit (depending on how lucky they get in locating the source of the problem). What's the most time-effective way to approach this task? P.S. Sorry for the long write up. Never been a man of few words.