If you're already maxing out the TSP then a SEP IRA and a solo 401(k) will have the same contribution limit (if not then the solo 401(k) will have a higher limit at lower self employed income). The SEP will mess up backdoor Roth IRA contributions if that's a concern to you. The solo 401(k) will be a problem if you ever have employees, has an extra annual tax form to file once it reaches $250k, and with the right plan will allow rollovers, so you can get rid of a traditional IRA to allow a backdoor Roth (again, if that applies to you).