As JimT says, the GRX sub-brake levers do exactly what you want – I put these on my gravel bike, albeit with drop-bar GRX brake-levers as the primary brake (which is what they are designed to work with) – having a set of these was the main reason I put Shimano parts on the gravel bike instead of SRAM. They work just fine, a bit of a pain to bleed, but they are pretty neat. I’d imagine they would work with a Shimano straight-bar lever too.
Anyway, there is a thread on here about them, which is probably best for this discussion: