Context is a Canon EOS 60D with Tamron 18-270 and Canon 10-22
1. My camera sometimes has focus issues especially in anything other than perfect daylight. It will cycle through the focal ring to one extreme, then back to the other, then display a red rectangle overlaid on the screen. From memory, this issue didn't always happen, it's only been a problem for about 4 years. Is there anything I can do to fix it?
2. Many photos I take seem to not be crisply in focus. Pic related on the Tamron lens is wholly not in focus and has noticeable visible fringing even near the centre of the image. I took this photo about a year ago and I couldn't tell you what I was supposed to be focusing on.
3. Leading on from 2, when I take photos I have difficulty telling on the camera's screen if they are in fully in focus or not owing to its small size, and I'm not confident enough with setting manual focus to be able to reliably focus it this way.
4. Many photos I take come out dark unless I take them under sunny blue skies. Even when I heighten the ISO and use a longer exposure time, they still seem muted. Do I just have to increase the ISO even more and edit out the noise later? I can't lower the exposure time too much otherwise they end up being blurry.
5. The chart in this article https://www.bhphotovideo.com/explora/photography/features/lens-sharpest-aperture-sweet-spot
suggests that optimal sharpness can be obtained by using a few stops up from the maximum aperture. Searching suggests that the optimal values are f/8-11 for the Tamron and f/5.6-8 for the Canon. Does this seem correct? I'm confused about the difference between the lens's advertised maximum aperture and how it relates to the aperture I select on my camera when taking a photo.
6. Could anyone recommend a decent prime lens that isn't too expensive that could potentially solve the above issues? The purpose would be for general travel (not landscapes), so I imagine I can get away with anything around 35-40mm.