How to get more focussed images with the 550D

That may be a good excuse to get a new camera. When I upgraded from my Canon T2i (550D) and Canon T3i (600D) cameras, I decided to get a total of three (3) Canon T7ii (800D) cameras. I keep them attached with lenses; that's why I have so many (I normally don't change lenses). I was looking at the Canon 90D and the Canon EOS R cameras. I don't know if there's an advantage of getting mirrorless or not.

In your case, I would upgrade both the camera and lens (by getting one with a longer reach). I know when I made the decision to buy the Tamron 150-600mm lens, it was a lot of money. But after about five years of continual use, it was money well-spent. I take pictures almost every weekend and having the right gear like that is important.

One time I accidentally dropped by Canon T2i (550D) camera with the 18-55mm kit lens. The camera bag strap broke and the camera/lens/bag fell down on to the pavement. After that incident, I found that there was an issue with the autofocus. I was lucky that it appeared to be only the inexpensive lens, as when I put in another lens, everything was fine. So maybe something happened to your camera after all of these years, to cause some problem like that.

If you are on a tight budget, you might consider a used Canon T2i (550D) or Canon T3i (600D) on something like Ebay from a reputable seller. I look for the cameras with low shutter clicks (as reported). Last year I got two new Canon T7i (800D) cameras for about $500 each including shipping from Ebay. This year, I noticed the prices went up a lot. When I first heard about the pandemic, I decided to get two of them instead of one. I like this photography hobby too much, to not have the equipment I want. I was afraid that there might be a shortage and the prices would go up substantially.