You can use regular Shimano gear to cobble together a decent 1x set up.

A regular RD will work, but you will need to do things to make sure the chain doesn't pop off the crankset. Paul Components and K-Edge make a nice chain keepers that wrap over both sides of the chain to keep it on. Or you can use something like an FSA carbon chain guard paired with a K-Edge Single XL chain keeper.

And if you want to go budget, you can use an old FD and bend the cage to allow it to run the full range of the cassette without rubbing. I did this with an Ultegra 6600 set up for cross many years ago and it worked OK. (6600 shifters, 6600 RD, 6600 FD, and DA7800 crankset with just a Salsa 42 chainring on the outside of the crank.)

I am currently running a basic Rival RD, Apex Shifters, no FD, FSA carbon bash guard and K-Edge Single XL chain keeper. It's a bit smoother than the Shimano set up I cobbled together.