I get your point, but sometimes it's a bit worse than having elongated stars just on the edges/corners.
On another note, astronomy is a fairly new thing for me (compared to someone like Jon or yourself). You might know for a fact that something that costs 1-1.5k is "low grade gear". I don't. I do now.
When I re-entered this hobby twenty years ago I had an email exchange with some guy on the west coast, I was unhappy that my mount was jittery and unstable and he seemed to know what he was doing. I forget whether I was buying something from him or what. And I said: what does it take to get rid of all these problems I described and he said "Losmandy G11." So I look it up, $1800 back then, and laughed. That was more than mount, c8, tripod on the rig I was using then. The price seemed ridiculous. I would NEVER spend that kind of money on a mount.
A year or so later I had one of those G11s and I still have it and still use it. Great piece of gear built like a tank. The expense works out to about $100 a year.
Now there were issues even back then that the G11 didn't deliver the kind of tracking people needed for longer exposures and there was a lot of complaints and a lot of after-market fiddling. I wasn't involved because I don't image. At that time there was a general movement (among OEMs) away from mounts that delivered 30 arc second periodic error towards mounts that could deliver 5 to 7 arc seconds or better, with a smooth curve that an autoguider could compensate. With some redesign and improvements the G11s got to that point too (the new ones). And now people seem to be buying mounts that are so accurate they need no autoguiding at all.
That $500 mount came and went because it was awful. So I was out $500 for one year. The G11 cost $2,000 and has lasted 20 years which I never would have imagined. That's $100 a year. So you see, cheap is expensive and expensive is cheap.
There is such a thing as getting the low end of the high end. The G11 is an example, and at $2k to $3.5k depending on the choices you make. The imagers at NEAIC (North East Astro Imaging Conference) tended to speak highly of the Sky Watcher Esprit 80mm as a beginner's instrument ($1800) though lately I've read here about some complaints with the newer ones. So that's a good deal less expensive than a Tak. My friend bought the Esprit, removed the focuser, and put an adapter on it so he can use it with his $900 Feather Touch focuser. I don't know whether to call that a $900 upgrade or a $300 upgrade because he has the same adapter for two other scopes and can use the focuser on all three. Photography requires precise focus and people tend to be willing to invest in that. You're trying to make wavelength level adjustments.
Wolfgang Promper did images like this one using a c8 (before there was an Edge) and a Losmandy G11 mount. We'll call the total investment in mount and tube $4,000 which I think is high I bet it was more like $3,000. That does not include imaging equipment.
Note that very wide angle shot is achieved by expert creation of a mosaic of smaller shots at f/6.3. So he didn't bother getting a $3,000 apo with a field flattener. Here's a pic he did of M106 (galaxy) much narrower field.
The thing to "get" here is that at least in his early years he "solved" the corner problem by cutting the bad stuff out and putting together mosaics, and he was good enough at mosaics to put together images that many people who agonize over their corners in pics taken with high end refractors never manage to equal.
Now Promper has since moved on to other types of gear that might easily be worth $100,000. He made a point of achieving a high degree of mastery with mid-grade gear before moving up to the $20,000 mounts with $50,000 optics. (Which is really beyond any ordinary sense of high end.)
There are very few people with $7,000 130mm apo refractors who match what he did with a c8.
Since he did that work Starizona (and Arizona company) has come out with a whole array of gear for imaging with SCTs at very fast focal lengths which get you very fast exposure times. That means that the mount is less critical which brings down costs. You work out a system with them and compare it to, say, a Takhashi TOA 130mm on an Astro-physics mount, and you'll have a good sense of what-most-people-say is mid-level gear vs what-most-people-say is high level gear.
So you need to find people who are getting good results and find out how they are doing it. "One-shot color" photography also can make getting started easier and less costly.
Also join Astromart and sell the stuff that isn't working out for you. Get that money working for you in some other way.