So you're saying mempool.space will relay it to Luxor pool? That kinda makes sense

Next problem: how to create such a transaction in Electrum? It's been
years since I had zero-fee transactions, so I'd like to try the lowest possible fee

I am away from my trezor but I am willing to see if it's advanced fee feature allows for 0.9 to 0.8 or lower.
It certainly would be easy if it does.
I know
1.01
1.02
1.03
1.04
1.05
And on up work as I tested them.