Weirdly, when it came to the render settings, I was kind of missing the old presets because I was used to fiddling with them to get things just right. That said, I gave Optix and others a go and became immediately suspicious along the lines of "It's blurry... I'm sure it's blurrier... This can't be right...There's a grain on the nose, or is that my monitor?

Didn't like the denoiser before and probably won't use it much now either.

As for HDRI.... I've used a few Daz Iray ones, which I converted to sIBL using sIBLEdit and then dropped into EzDome and that's been ok. Only trouble I had with that was getting the sun colour right in sIBLEdit. (missing some trick I expect) Mate told me that the 16k HDRI's from CG Axis are fantastic but I've yet to try them. Essentially, I have stuck to the EZDome method most of the time but shvrdavid's HDRI Setup over in my Superfly thread (https://www.renderosity.com/forums/threads/2990609/superfly-help-and-examples-especially-with-respect-to-pbr-textures) is really quite fantastic.
Do get fireflies with some HDRI's and especially sundown ones.
Single biggest issue with HDRIs has been the scaling to be honest. Background houses and stuff looks enormous and off every time, so that I have fixated on Puresky HDRIs becaue other ones look bad.

Didn't realize DoF was broken in 12! :D :D :D  Bugger! And there was me using it and marvelling at how it was finally usable compared to Poser 10 etc... *dons "Dunce Hat" and goes to stand in the corner*

I have two of Ken1171s scripts already and especially the "go physical" one has been a massive time saver.