I can't get my head around why the transaction would get "dropped for good once any of its input is spent by another transaction."
I am the sender, and I've tried re-sending that UTXO(?), after resyncing the SPV of the decentralized exchange I am withdrawing from. This attempt to re-send is pending (on the interface of the exchange), and can't be found on mempool.space. I imagine that the attempt to re-send is pending because the previous transaction (the one in question, that is inbound to the Electrum wallet) is still "in the network". That is why I struggle to understand how the transaction would get "dropped for good once its input is spent by another transaction".
I'm curious to understand what I am missing here.
If your re-send successfully propagates then the original transaction will disappear forever from every mempool. Since the re-send can't be found on block explorers it could mean that your original transaction didn't have RBF enabled and nodes still have it in their mempool. This will result in the new transaction getting rejected because it conflicts with the original.
Your best option is to connect to an Electrum server that is able to see the original and use CPFP to bump the fee.