I work on industrial machines, and this would be my approach, one step at a time (FWIW):

Make sure the top is threaded correctly, and the top thread is lifting off the spool with no resistance. I found this threading diagram online. Run the thread through all the holes in "3" on the diagram in a spiral (for now).

Take apart the top thread tension plate assembly and wipe it clean and put it back together.

Check that bobbin tension again. Since it's a used machine, take the tension spring completely off the bobbin case and clean out any fuzz. Put a bobbin in and pull the thread slowly, it should be a nice even resistance (you can adjust later).

Make sure you're not using thread that's too large for your needle eye. It should pass through very easily, with no burrs. "Floss" it to check, should be smooth and easy, with minimal thread damage.

Oil the bobbin basket raceway (where the thing that holds the bobbin touches the hook). Rotate the handwheel by hand through a couple stitches, and make sure the top thread passes around the bobbin smoothly.

If that doesn't work, take apart the hook basket (the thing the bobbin case sits in, probably 3 tiny screws and a curved piece of metal) and make sure there is no fuzz underneath. Clean it up, put it back together, put a couple drops of oil on it.

Your hook timing is probably fine if it's not skipping stitches or getting jammed. Sounds like a tension, needle or top thread issue from your description.