Probably depends on the CoL where you plan to make that living. Working FT (40 hours a week) I could easily clear 150k just seeing clinical patients. However, I enjoy the IME work, and that just so happens to pay several times the amount I make hourly compared to clinical work. You can make a living either way, one just happens to pay substantially better.
This has also been my experience.
I'm now 70% legal & 30% clinical, which is around the split I want to keep. This has freed me up to do some pro-bono work (clinical & consultation) and more volunteer work (not psych/clinical).
As for billing, I tried doing my own billing, and it just wasn't worth it. I admittedly only do workers comp and legal work, but WC was a nightmare to try and figure out on the fly. I never did my billing when I saw Medicare, so it was a steeper learning curve that proved not worth it to me.
Legal billing is easier, but more recently I decided to have my billing company handle that too. I pay them 8% (I've seen 6%-8%), which sounds like a lot, but it's a drop in the bucket compared to what I would have lost if I relied upon myself. It's important to go w a biller who knows Neuropsych billing bc you want/need to be credentialed on the medical side. Being credentialed through behavioral health anything will decrease your rates and add more hoops.
Commercial insurance tends to pay okay to poorly, and they definitely have some added hoops. A couple jobs ago (where I took limited commercial insurance), I made it worthwhile by billing 80%-100% of my cash rate, but it takes some more work up front. I negotiated with each insurer and often did 1 off contracts paid at my cash rate, so those were more reasonable. I knew the market, knew there was a shortage of Neuropsych, and I got them hooked on using me 1 contract at a time.
Insurers will rarely talk about one off contracts where you agree to see a specific patient for a negotiated fee/rate bc they want you to join their network and accept scraps for payment. You can always negotiate rates (except Medicare), so it pays to do your research and hold out for top dollar.
ps. All of this is written as someone 10yrs post fellowship, exp w multiple private practice settings, and with a biz background. Don't expect to run a largely cash practice coming out of fellowship, but it can be done of you can build a reputation and find a good niche.