I don't know how you got it to work because it didn't work for me. After I switched to the server, I saw the usual notification that the server won't broadcast my transaction when I set the transaction fee below 1 sat. I restarted Electrum a few times but that didn't work either. When I closed the send tab a few times and opened it again, it allowed me to enter a few below 1 sat/vByte without that notification popping up and the "preview" button became clickable. But what I noticed was that the equivalent amount in USD was as if I had set the fee to 1 sat/vByte. The wallet didn't lower it regardless if I set it to 0.7, 0.5, 0.3 sats, etc. There was also a notification next to the total fee that said "xxx amount of sats will be added to the transaction." It always added enough to make the fee 1 sat/vByte.

Are you sure you didn't see the same and you just think that you broadcasted the transaction at 0.3 sat/vByte when in fact you didn't?