I know how you feel. Years ago I bought and refurbished some inexpensive guitars - primarily to learn how to work on guitars. Since then I sold them on eBay. Fortunately I had a friendly luthier who gave me some tips. Since then there are tons of youtube videos.
You might just need a set-up. This is non-invasive and a good place to start. A tech could easily charge $75. So with even a small guitar collection, it can be expensive & time consuming to use a tech for setups. A ruler that can clearly read in 32nd's of an inch and/or millimetres is needed. Charts are available online to convert fractions of an inch to decimal to millimeters.
I had to learn to accept that it's a time-consuming, intricate dance of: string height at the nut; at the 12th or 17th fret; neck relief; and finally how the tremolo is set. Adjusting one thing effects the others. Also these things are subjective, within a range of operating parameters. Factory setup specs can be googled, which likely are good enough. Note that because of neck memory, in week the neck may revert to its old position especially re the relief setting. Multiple adjustments over time may be needed.
If a "simple" setup doesn't fix the problem, then more invasive work may be needed. This may require adjusting the nut (from the bottom, and/or string slots); or a fret dressing. It may be that some fret are not fully seated, or are lifting at the ends which can be seen & felt by inspection.