Should I send all of my usdt and busd into my desktop electrum wallet for long term savings?
Electrum onky supports Bitcoin.
For altcoins, I will advice you to go for
open source hard wallet, or if you are able to get a completely open source wallet that is recommended.
I noticed people using altcoins may not have option than going for a close source wallet like Trustwallet which support such coins. But not Worth going for close source wallet because the wallet source code is not publicly available.