I am developing a web app that needs to check balances and send transactions for BTC, LTC, DOGE, XMR, and hopefully others. Is there a good tool (ideally Python) that can be used to check balances and send transactions without sending the keys to an external API, but without running full nodes for each currency?
There's no. You must make it by yourself. As far as I know you will always need to sending keys to an external API.
Any suggestions?
Are you looking for something like bulksender of ethereum? Those coins have different sha and you must work by yourself to create tools for any coins. I'm not sure if it can be used for multiple chains.
I'm looking for something similar to a command-line Electrum program, where a full node isn't required. The kicker is that it should have a unified interface for multiple cryptos.
I never heard any other similar service, it seems like that electrum was the only one. I'm only seeing this feature on coin98 wallet but I rarely seen the platform was providing this kind of service for native coins like doge, BTC, ltc and other