I think that mempool.space is the best option, I like its visual aspect and nice interface. Personally, I would never trust Electrum to decide my transaction's fee because most of the time you'll be overcharged. On mempool.space, you can see the recently mined blocks and upcoming blocks, you can see the fee range in each block, all of these give you a better understanding to decide fee for your transaction.
And Electrum wallet isn't the only Bitcoin wallet that has an adjustable transaction fee, I use Trust wallet and you can also easily adjust your transaction fee in this wallet for Bitcoin..
Trust wallet is close source. You should use open source wallet.
Trust wallet is also owned by Binance, so a big no no.