How much 'better' is PM vs Reg-t for shorting puts using margin as collateral?

I've just recently learned about PM and want to learn how beneficial it would be for my use case.

I'm essentially trying to use BOXX (an ultra low risk synthetic T-bill) as collateral for shorting puts. Brokerages don't allow me to use BOXX as collateral directly since it's not a cash equivalent, so my strategy is to use margin cash secured by BOXX to use as collateral for shorting the puts instead. I don't pay interest on the margin cash until the contract gets assigned, meaning I can close the position before expiration or roll the option forward to avoid paying margin interest.

I'm currently approved for reg-t margin on Vanguard at 50% LTV. A risk I see with this strategy is Vanguard changing margin requirements while I have open contracts. Let's assume I'm getting 50% LTV for margin cash, and 90% of my available margin cash is tied up as collateral, if Vanguard reduces the LTV amount to 35% during a market downturn, that would not be a fun situation. Ironically, the moment where I want the most access to margin (during a market downturn) is the most likely moment they will limit my margin access.

It's also annoying how the maximum margin cash they'll give me is at 50% of the value of my BOXX. Like if I buy a strike on XSP at $700, I'd need to put up $70,000 of margin cash as collateral, but the chance I 'need' all of that $70,000 as collateral is basically negligent, XSP would have to drop to $0.

Does PM solve these issues? I never want to trade over the value I have in equity in BOXX to 'over-leverage' myself. Like if I have $100,000 in BOXX, I would never borrow more than $100,000 in margin, even if I technically could.

For examples sake, let's assume I have $300,000 in BOXX in my account. 50% LTV would on reg-t margin gives me $150,000 I can use as collateral. At a $700 strike on XSP, I can trade 2 contracts max. How would this look different on PM?

Let me know you'd like me to clarify anything in the comments. Thanks for the help.