This is an issue of the past for me since I started using Real Debrid with Kodi and Stremio.
I have a good backup amount in 1080p in Plex (physical and debrid symlinks), but when I want to experience 4K films properly I head up to Stremio or Kodi.
You're not going to like this, but I have been encoding my own B-tier collection in 1080p at roughly BluRay quality with HDR & Uncompressed audio. A-Tier movies stay 4K Remux, but the B-squad gets the downgrade (but I'm not willing to give up HDR / Atmos)