"I have set up my taxable brokerage accounts with margin. I did this so that I could buy shares without having cash in the account. If the trade happens I transfer funds via ACH."

My experience at both Vanguard and Fidelity is that they permit me to trade immediately after I transfer funds from my bank via ACH. They do not wait for the funds to clear/settle before they are available for me to trade. I suspect that there would be some hold if the amount is large enough relative to the overall balance of my portfolio.

Lately, I have been keeping most of my cash in treasury bills at Vanguard. So if I need funds to buy stock, I could sell t bills and immediately have funds available to transact. When t bills were yielding nothing, I tended to keep more money in my credit union which was paying 0.5% back when treasuries were at 0%. But I rarely kept more than the FDIC limit at the credit union.