Every single contribution, loan or withdrawal must be in the form of fiat. This means if you want to take a loan/withdrawal, the trust needs to have enough liquid assets (stocks, bonds, crypto, cash).
I'm happy with the provider I chose so far. They handle all the paperwork, IRS filings, etc. for a flat $49/month fee. All I had to do is sign a few papers that they prepared, open a bank account, open an E-trade account, then open a Kraken account. No transaction fees. I can PM you my referral code if you like.
The beauty of a Solo 401(k) is that it's truly 'solo', which means you can invest in whatever you like while your business partner can choose their own investments. So I can see it working for your situation. Whenever you think ASIC or GPU prices are too high, or a bear market is coming, you can contribute your share of profits to the Solo(k). No need to buy more equipment if you want a tax deduction.
Instead I'm developing this Stratum software that can distribute hashrate among many customers, switch pools, boost revenue with profit switching, etc.