is there an (AV receiver) type device that doesn't have amplifier/preamplifiers?

The WIIM streamers, or any other streamer, resides on your home network, either wireless/wifi or wired. You can control the streaming app from your phone or a computer. In the streaming app, such as Spotify, you select where the music is going to play. You would choose the WIIM streamer, select the song/track/album and hit play. The music files are played by the streamer and it is output to whatever you have the streamer connected to. In this case, you would have the WIIM connected to your DAC, and the music will play on the speakers connected to your integrated amp.

If you get a streamer and want to use your DAC, you'll just need to insure you have an available input on your DAC and you have a corresponding output type on your streamer. You are using an optical connection from your cable box to your DAC now, correct? You'll need another optical input on your DAC if you need to use an optical output from the WIIM Mini. The WIIM Pro has a coax output as well as optical, so you could use a digital coax cable instead of optical. Will work exactly like to optical connection.

@mainlymusicman  - Awesome! Glad it worked out and you have a simplified system. If you want to try another digital source, of course you can stream from your phone or computer via Bluetooth. However, Bluetooth is not a lossless format and you will have somewhat diminished sound quality. Might be good enough for you. You could also stream via a dedicated streamer such as a WIIM Mini, or another higher-end WIIM streaming device. I use a WIIM Mini and stream Spotify. Spotify is also not lossless, but it sounds good enough for me. I also just bought an inexpensive $105 SMSL PL100 CD Player/Transport that I'm going to run through my DAC as a test. If it's as good as my expensive dedicated CD player, I'm going to sell the CD Player.

Yes, from my experience, the HDMI and optical outputs should be active at the same time. Any DAC that has an optical input should work. Both of the ones you listed should be good. For less money, the Schiit Audio Modi  is only $129 and is very good and from a very good company.

The only potential issue I can imagine is audio out of sync. Used to be an issue many years ago. Have not heard/seen mention of it in many years.

If you are only using your two front main speakers you don't need the AV receiver. An optical cable connected to a DAC, then to the Plinius will work just fine. I've done it many times over the years. You may need to go into the setup menu on your cable box and choose your optical output and/or 2 channel audio. But somewhere in your current chain it is downmixing any multi-channel audio to 2-channel stereo, or else your missing a lot of content (such as discrete center channel dialog and/our surround effects). But it's probable set up correctly now and down-converting to 2-channel audio somewhere in your current chain.

You can get a decent DAC for $150 or less. Buy it through a source with easy returns, such as Amazon. Return it if you can't get it to work for you. I'm confident it will work.

My current cable box, and every other cable box I've had in the past, had L&R analog audio outputs as well as some type of digital audio output (either coax or toslink optical digital). Your cable box likely has some type audio output that is independent of HDMI. If it does have analog R&L RCA outputs or some type of digital audio outputs, you can connect the HDMI cable directly from your cable box to your projector. You would then hook up the L&R analog outputs from the cable box to one of the line level inputs on the Plinius, or connect the digital output (coax or optical) to a DAC and the output of the DAC would connect to a line level input on your Pinius via RCA L&R interconnect cables. A DAC is a digital to analog converter. It converts a digital audio signal to analog so you can then connect it to one of the line level inputs on your Plinius Integrated amp.