Electrum Server running. Now what? I finally found time to build a full bitcoind node and run electrs (not Electrum Personal Server).
What things can I do now? I know about only these:

Point my Electrum wallet client to my electrs, even over Tor (main-use case).
Searching here, some people write scripts or custom apps to talk to their Electrum Server (ElectrumX, electrs). I assume for block explorer type queries.
My own BTC RPC Block Explorer would query my Electrum Server (and bitcoind?)

Questions:

Can non-Electrum wallets talk to Electrum Server? Is there an easy way to find which ones?

I'd like to make my own mempool observer app. Maybe I would query Electrum Server, or is bitcoind better?

In what situations would I call Electrum Server's api vs. calling bitcoind?
Said another way: What API data does bitcoind provide vs. Electrum Server, and is there any overlap in info they both provide?
Here is the protocol: https://electrumx.readthedocs.io/en/latest/protocol.html. If get_mempool serves your use-case then you may use it. You may need to experiment with it to find what is more convenient with Core.
Some apps might be using the Electrum protocol under the hood. Only Electrum allows setting the server manually.
Electrum protocol has blockchain.scripthash.get_mempool which doesn't help you build a mempool observer. You need to connect to Core directly then