I'm going to offer my opinion without intentionally being degrading or disrespectful.
I am not a TPA, but I work with TPAs and actuaries in the course of my business. The TPA business is highly specialized. While many are, in my experience I've found the gambit between those that are very good and those begging to be sued, and the latter don't last long. All reasonably successful firms have YEARS of experience.
Though not a TPA myself, knowing what I know I would never consider just jumping into the business for whatever reasons you have. Although credentials and education help, you can quickly find yourself overwhelmed in an intricately complicated subset of tax law. You will also need a good understanding of the Employee Retirement Income Security Act (ERISA), which is a bitch of a law to understand (hence my username). It takes quite a bit of experience, hands on knowledge, and field expertise before you can reasonably be expected to be competitive.
Some of my CPA referrals for retirement plan clients come from CPAs who discover they don't understand the business, and most of those referrals come only after they have talked to their friendly competitors/colleagues who refer me. Most of my referrals come from competent CPAs who KNOW what they DON'T KNOW.
The best TPAs I work with have years upon years of experience, with the back-end personnel to support the business. They often have ERISA attorneys on staff or standby. The have actuaries on staff or outsource to those that can handle that business. The typical TPA firms I work with have at a MINIMUM of a dozen staff with the education and background in the business. I do partner with some with less, but those are generally recognized industry experts.
As someone who recommends TPAs, I would NEVER, EVER recommend a TPA business just getting started with no experience. And when I find a client who has a "two or ten plan tony" TPA (rare) or advisor, I'll take your business away from you to one of my expert TPAs before you know what hit you.
I don't mean to be discouraging, but I'm simply relating the realities of the business. If you are serious, then:
(1) Learn about ASPPA and their credentials, and what it means to be a TPA;
(2) Determine if that fits in your business model and if you have the bandwidth to pursue a new, complex education while servicing your existing clients;
(3) Assuming (2) makes sense, get the necessary credentials from (1);
(4) Spend a significant amount of time attending conferences (NAPA, ASPPA and others) and studying webinars, white paper, blogs (like this one), and other educational material provided by more sources than I can mention (do your own research);
(5) HARD - Figure out how to partner with a small but experienced, reputable local TPA that will review your work;
(6) Build up your expertise and consider merging with (5) or relocating, depending on what your non-compete looks like;
Otherwise, you will just be a small fish with an "eat me" sign in an oceans of sharks.
Hope that helps!