Brokerage for 401k LLC (self directed)

I have a 401k that wholly owns an LLC that I use for alternative investments. However banking is a bit tricky as most major banks don't want to deal with these type of accounts - it doesn't fit into their checkbox idea of a retirement account or a business account, though it's just a business account.

I have a checking account for the LLC with Titan Bank and I believe Solara Bank does this as well. But this means that when my money isn't invested in alts, it's sitting in a 0% interest checking.

What I'm looking for is somewhere I could set up a brokerage account with this 401k LLC structure. Any suggestions?