I have a 4K JVC NZ8 and a Panasonic 'Kuro' 65" 1080p Plasma, with an Oppo 203 & various 4K streaming boxes, with most 4K components gradually being added post-2021.
Full disclosure though - it's all connected via a Lumagen Radiance Pro, which may have an influence.
When I was working in the USA from 2018-2020 I started buying 4K Blu-Rays to future-proof myself, but always the versions with the additional 1080p disc. I still seek-out these double packs which are of course more expensive, but they're always available from somewhere -
Blu-Ray.com seems to be the best source of information regarding versions of discs globally, and these days it's not difficult to get discs sent from e.g. the USA, or from the UK, or directly from the Studio's website, etc.
It's also worth remembering that sometimes some of these dual-format packs do have different versions of the films on the 4K or 1080p, plus it's the 1080p disc that usually holds all the extras.
When I watch 'anything' on my 1080p Plasma (apart from some crappy SD feed) it all looks really very very good - regardless of whether it's a 4K source or 1080p - and until April 2023 when I replaced my Sim2 1080p Projector with the JVC, the picture was still really good on the big screen.
My understanding is that if a Player detects a 1080p source, it should still apply any appropriate processing (HDR10 / WCG) but then down-sample the resolution, but I may be wrong.
On
the Lumagen thread on AVSForums recently, some Studio Mastering Experts have recommended the Panasonic 820 / 9000, the Reavon & the Magnetar for the best performance.
Hope that helps!