4. Run Electrum server and configure your Electrum only connect to your Electrum server
"4. Run Electrum server and configure your Electrum only connect to your Electrum server" - how it can help me to use many different wallet.dat files?
You could export private keys from each wallet to each electrum wallet. Not practical, but it's one of possible options.
I don't know how would this help, sorry. ElectrumX needs Bitcoind and the whole blockchain.
There are other Electrum server implementation which allow you use Bitcoin Core on pruned mode.
Example : https://github.com/chris-belcher/electrum-personal-server