I was wondering what you meant by Struggling with the number of themes in my work
Because I think of themes as coming up organically and not being something you explicitly write.
But now I wonder if you simply have too many ideas. No such thing, right? Unless you get bored and move on to the next idea before the first one has reached fruition.
If your scenes are truly that good, then you know you can write. The problem is sustaining it over the course of a novel. I know from writing my own novel that sometimes you get to a point where it is tedious and you feel like you're slogging through with no end. I'm at a place myself where I've been stuck for a couple of months figuring out family structures, names, and ages of a group of people. This was easy and fun when they were contemporary and done from scratch. It's been hellish doing it with this different group for a historical time period using an existing work and adding to it. I have to make myself push through it and finish it already.
I could just keep writing and finish the book. I have it all in my head. But without the part I'm struggling with, it won't be grounded. It will always be off and I might not even know why.
If you've written several books in 20 years (and wow, most people never complete one book so yay you!) that have a couple of good scenes and the rest is meh, it makes me wonder how much of the boring stuff you've waded through and how many rewrites you've done. Maybe I'm off here. Maybe the issue is structural. That you can write good scenes but have trouble tethering them.
Either way, it's time for outside help. A writing workshop (the kind where you spend 2 weeks on a farm or something), a novel writing class (full semester), a professional editor who's not going to pull punches. Somebody who can figure out exactly what's wrong and how to fix it.
Maybe one or more of your existing books can be pulled apart and re-written. Or maybe you need to start over with new skills. I don't know.
If you've been doing this for 20 years without success, I think it's beyond what some good introspection and online advice can do. The only analogy I can think of is therapy. If you were struggling with a deep-seated personality issue based on past events, introspection and the occasional support group sometimes helps but can only take you so far. It's time to hit the couch and find out what makes you tick. Only in this case, the therapy is for your writing, not your mind.