installed a halo ground system here and converted to an isolated ground system in the shack. Yes, I am familiar about how to properly do this, my telco background where we used an integrated/isolated/data return ground system came in handy here. I know there were some EE & electrician types that were about to jump down my throat on that one, cool your jets! I used 9 inch wide pieces of copper sheet for the ground system inside the shack with 4 inch wide pieces for risers and 2 inch wide pieces for connection to gear. All receivers operate via AC isolation transformers with only the isolated ground connection.
So long as you re-derive the neutrals and bond the ground/neutral at the secondary of the transformer, you're OK insofrar as the NEC is concerned. Be advised you need to do this for ALL equipment hooked to that antenna/ground system - xmitters included.
Has anyone else noted a general rise in the noise floor in recent years?
Yes. The advent of switchmode power supplies and SCR power switching gear appears, at least to me, to be the big culprit.
Do the modern Japanese HF DSP rigs do away with this problem completely?
No. The noise DSP in my FT1000MP MK5 certainly helps a bit with the S-9 power line noise I have but that's at the far back end of the receiver. The noise still reduces the sensitivity and thus the S/N ratio. Worse, the DSP in that radio only works in SSB/CW modes, not AM.
My location was very quiet, noise-wise, until a couple of years ago. My lake place, with a "shaded dipole" below the rim of the mountain and threaded amongst saplings, is very quiet but not entirely noise free either.