Previously i had the same issue you are having when trying to install NEO, I had BTC and ETH and room for nothing else. Personally, I deleted the Ethereum App, it seems to be pretty large. I have BTC, ADA and NEO apps on my ledger now.
It really is super simple to just delete an app and reinstall it when you need it. Unless you use ETH transactions a lot I’d remove that one, I keep NEO installed for claiming GAS.
Yoroi makes it really simple to get up and running with Cardano on Ledger, and you can still see your balance/transactions etc without the ledger connected, so i’d still be tempted to get my ADA on the ledger anyway if I were you. If you don’t make many ADA transaction, just uninstall and add ETH back onto your Ledger.