One of the folks who works at M1 Finance pitched me on the following, for folks who have similar cashflow characteristics to the median HNer. (They've essentially productized this, and with slightly more work you could get it out of Interactive Brokers. It's hard to do with most discount brokers due to prevailing margin rates.) It captures most of the advantages of the "hybrid" account and is actually available in the status quo, which is an advantage.
Deposit your paycheck. It is automatically invested. Spend on margin; repay it the next month on payday, when you repeat the cycle. Change this only if you experience long-term unemployment, which you a) probably will not and which b) savings is the least effective financial product to prepare you for it.
Their theory is that the ~3% impact of margin is less than the fundamental return of an equity-heavy portfolio measured in small increments over a long period of time, so you should prefer to be perpetually ~100% invested in your taxable accounts rather than "materially less than totally invested", which is what most well-paid professionals are given usage of a checking account and poor decisions embedded in their financial products. (e.g. If you have $50k net worth, $8k in post-tax payroll per month, and are saving $2k per month, you're no higher than 86% invested and if you are following common financial advice you with a six month buffer in a non-investment emergency fund you are ~24% invested.)
This sounds strikingly plausible to me, for what it is worth, again contingent on one being a well-compensated professional with predictable cashflow in and out and a temperament amenable to discipline and commitment. It is not not not not the consensus of advisors to the American middle class.