If you are worried about sound quality I wouldn't settle for DD/DTS over SPDIF. A 568.2mm with an HD621 is night and day better than a 568.2mm without a HD621. You may also have the problem of having a 4K HDR TV, that a HD621 can't handle, and then you get into all sorts of fun having the HD621 purely as a device to get MHR out of HDMI and into a 568.2mm or 861.

But for incremental upgrade path, perhaps a 568.2mm with a simple SPDIF audio extractor (like the blusound), would be a good first step. Then add a HD621, with the blusound now splitting the signal to the TV (for 4k HDR) and to the HD621 (for audio). And then perhaps an 861v4 for room correction.

Cave: Trinnov Altitude CI-8; Oppo 203 Vanity HD; 6xDSP5k, DSP5kC, D1500; Sony Z9D, PS5 Pro; Apple TV 4K
Living: Wiim Pro; Rega Planar 1+; 2xDSP5k
Master: Wiim Pro; 2xDSP3100
Kitchen: Bluesound Flex stereo pair / Amazon Echo Studio
Deck: Wiim Mini; Random Chinese Class D; Cambridge Soundworks Outdoor 52s; Polk Sub;
Garage: Wiim Pro; 2x5k