I had a thought the other day (dangerous I know).
I watched a YT video where he used a large paint strainer bag as his BIAB, pretty sure his pot was 8gal. Steeped with 3 gal, splarged with 2 or so gal right in the pot.
My only thing is, I would think the pot would be hard to keep the temps up in. You can't have your burner on when steeping as you don't want to burn the mash, so it would bleed off heat pretty quickly.
Why couldn't you use the best of both worlds. BIAB in a cooler? No need to modify the cooler for a mash tun. Pour in your hot water, grains in the bag, put the bag in the cooler, close and wait. Then splarge with however much you need. Once done, drain it all into your brew kettle and go. You could potentially use more grain that you could fit in the 8 gal brew kettle. Likely already have a cooler and wouldn't have to modify it, this upsetting SWMBO, or spending money on another cooler.
Just spitballing here, but wouldn't that work?