I don't believe that you need a TPA if you're just doing a solo401k, posting in part to hear if I'm wrong.
I used a TPA for my previous contractor employment, it was my only source of income, and I set up an S-corp, paid myself only a W2, and did a solo401k plus DB/CB plan, but my understanding was that it was the DB/CB part that really required the TPA.
I currently have a W2 job, and a 1099 sidejob, and for the latter I just set up a solo401k through E-trade this year, with no TPA; I don't plan for this account to reach the level necessitating filing 5500s, at least for the foreseeable future.