But it didn't reject it, and it also didn't send at all at my specified sat/vB. Instead, it doubled the miner fee for the batched tx's at 2.01 sat/vB. Which is essentially more exoensive than sending separately, without batching.
Your first transaction was made with the absoulte fee of 141 sat and your second transaction was 171.25 vbytes in size.
This means that the minimum required fee for the second transaction was 313 sat and the minimum required fee rate for that was 1.83 sat/vbyte.
The second transaction could be made with 31 sats less in fee.
So I decided to add an other one to the batch. I decided on a 1sat/vB fee for that third one. But dumb Electrum basically increased the miner fee from 2.01 sat/vB on this one. Basically not increasing the fee at a meaningful level.
Your second transaction was made with the absoulte fee of 344 sat and your third transaction was 337.75 vbytes in size.
This means that the minimum required fee for the third transaction was 682 sat and the minimum required fee rate for that was 2.02 sat/vbyte.
Your transaction couldn't be made more economical.
I don't like this one bit. I want the old Electrum I was using a year or two ago. This is regression.
Again, nothing has changed with how electrum caculates the fees.
Please read
BIP125 requirements.
The smaller the replacement transaction is, the more fee rate you have to use for that.
The larger the replacement transaction is, the less fee rate you can use for that.