I was looking for the second HW wallet (I already own Trezor One for BTC, ETH) to hold my alts (like ADA, ATOM, NEAR etc.).
I thought to get Ledger nano s+, but are there any open-source alternatives for +/- the same price?
Just curious why don't you just get another Trezor One? It fit's the requirements of open source and it's in the price range. It also happens to be a great wallet. The only drawback I find is it's not compatible with Monero's official wallet.
The other question that comes up, is why do you need another hardware wallet, just as a back up? If it's just so you can have another set of addresses, you can do all that with one hardware wallet by setting up multiple Bip39 passphrases and/or derivation paths.
basically the main purpose is to hold coins and connect to wallet once a month to check the balance.
There's no need to connect your hardware wallet to check your balances. All you need are watch-only versions of your wallets.