Bitmax:
1. The testnet address I've been trying to get tBTC core to send to is my tBTC mycelium app. However, I've now found that both Wasabi Wallet, and an online testnet faucet also reject the bech32 testnet address created by my phone's mycelium tBTC app. (So that's on you, Mycelium).

2. I determined that my tBTC core fails to propagate if I use
$ sendtoaddress [address] [amout of tBTC]
but, if I use:
$ sendrawtransaction [raw_transaction]
then that propagates right away.
Interestingly, the same failure to propagate occurs if I use bitcoin-qt -testnet.

Any chance this is due to my Linux firewall settings?

My bitcoin.config has already been nearly identical to what you've recommended:

testnet=1

listen=1
server=1
proxy=127.0.0.1:9050

[test]
prune = 5000
regtest=0
dnsseed=0
upnp=0
setgenerate=1
blocksonly=1

addnode=104.237.131.138
addnode=151.80.205.132
addnode=192.155.82.123
addnode=74.220.255.190
addnode=80.100.203.151

maxconnections=15
minrelaytxfee=0.0001