Is a Roth IRA only able to convert to a self directed IRA, rather than a self directed 401K?

That said:Some ideas-Depreciation bifurcation, IRAS,401ks, ESAs, HSAs, defined benefit programs, do deferred maintenance before end of year. 

I have significant amount in my IRA, but am not of retirement age.

I’ve seen parts of this question answered, in previous threads, but looking for a more comprehensive answer/advice.Quick background, I currently work a W2 job, have my real estate license and practice general brokerage on the side, and will be using some excess cash I’ve been able to save to invest in small multifamily (1-4 units) real estate.Aside from those factors and getting to my question, I have a 401K from a previous employer, pension from that employer, and small personal IRA (Traditional) which I would like to rollover into a SD IRA or Solo 401K (if I qualify) to passively invest in MF Syndications.Total rollover would be approximately $70K.What would be the best way to do this, SD IRA vs.

I quickly discovered the Bogleheads and FIRE communities and became an index fund investor three years ago, maxing IRAs and 401(k)s and all that fun tax optimization.

Consider trusts, LLCs , c-Corp, tenants in common, limited partnership, IRAs, 401ks, HSAs, maybe offshore entities, etc. 

think about Ira, 401k, HSA contributions to bring taxes down further.

While a Roth 401k can be transferred to other Roth qualified plans and Roth IRAs, a Roth IRA can only be transferred to another Roth IRA.

I already have a Roth IRA with stocks, and I know there's no good way to transfer that money out.

The IRA would own the LLC, but you could act as manager for the LLC and conduct the activities of the LLC.