I've been reading the other comments, and while people are encouraging, their comments seem a bit too "you HAVE to learn CAD." You definitely don't NEED to know CAD. I made basic parts and modifications in the slicer for nearly a year after I started printing, and it worked really well. However, if you are considering learning a full CAD program, I have two pieces of advice.

First pertains to if you are working with functional parts. Then you are talking about a parametric CAD program (fusion, onshape, FreeCAD, etc.). In this case, I think it's worth learning for you, and it's not as hard as it seems. You say you have SketchUp experience, so I'm assuming you have decent spatial reasoning. I know someone with no tech literacy nor programming experience who learned a CAD program very well in less than a month of following tutorials in her free time. Just give it a try, and it's a skill you'll be happy to have.

If you are working with cosmetic parts like miniatures and helmets, then you might need to use something like Blender. Admittedly, that can be even more challenging than the other CAD programs I mentioned. However, if you spend a few hours learning some basics from YouTube, you should be able to do fundemental things like fixing holes.

Hope this helps. Good luck with your printing endeavors!