Payment processing and Hierarchical Deterministic wallets

Hi! Not sure if this is the place to ask about development with ethereum.I want to add ETH (and ERC20 tokens later) support to my self-hosted open-source cryptocurrency payment processor. We currently only support bitcoin-based coins via electrum wallet. That way the processor can run on any server with minimal resources because of electrum's Simple Payment Verification and connecting to many servers at once, validating their answers.

For ethereum I was searching for something similar to electrum to just use it directly and wrap their APIs, but found none. Also most wallets seem to be just for one address only.

As for the networking part, I decided to use geth in light syncmode and use it from web3.py, I didn't find any better way.

But as there isn't really something similar to what electrum offers, but for eth, I decided to add a subset of electrum commands but in eth ecosystem with my own limited implementation.

As ethereum is accounts-based and not UTXO-based, I wonder if it even makes sense to use a new address for each invoice? If so I would probably implement something similar to gap limit system in electrum, watching first 20 addresses, then going through history of txes discovering new addresses and scanning 20 next addresses until there are 20 addresses with no transactions. I've just found another issue with this: there is probably no way to at least get transactions count for an address without creating an additional index?

Maybe there is a simpler way, I've heard I could create a smart contract and deploy it many times (but it probably involves some fee, and in my software users only enter xpubs so I can't spend anything), and the contract just forwards to one address. The problem is that I also want to provide some API for developers compatible with electrum one.

As for the actual checking payments, there isn't any direct way to listen for new transactions on address, so I will probably just periodically run every 5 seconds, checking all pending invoices and balance on invoices' addresses. Is it a good way?

Sorry for the long list of questions (: