With the current mortgage rates, does it make sense to self fund using equity backed synthetic loans?

Currently the best mortgage rates are around 6.2%, which is quite high compared to the period of low interest rates that we had.

As someone who has a large diversified portfolio of equities and bonds, I've been considering using SPX box spreads to self fund my mortgage. Currently you can borrow at around 4.2% for a 5 year fixed loan, as can be seen here. You also don't need a down payment, and neither will I have to sell stocks to get the down payment which would trigger capital gains taxes, so that's another plus.

The only risk is the risk of margin call from your broker, but I'll be keeping the loan amount at 20% of my portfolio value, so the market would have to fall over 80% for me to be margin called as per current maintenance margin requirements at my broker. I would then pay off this loan with the rental payments from the tenant, and finance it with a traditional mortgage once the rates are better.

This seems like a foolproof strategy to save %2~ additional costs on a mortgage. Has anyone done something like this before?