How to properly combine PSBTs for CoinJoin?
I am very embarrassed, just circling back to share, the above workflow works perfectly. My problem was I had an incorrect vout value in one of my inputs. Basically I was trying to spend money I don't have (/facepalm). As soon as I corrected that, everything worked flawlessly.

Here's my multi-wallet.dat file Coinjoin :)
https://live.blockcypher.com/btc-testnet/tx/537d4f9c49e06916c8f74ad53e52c3389f1654f4dec1014e2335c7ad49e268ab/