Bitcoin Core includes options to reduce network traffic. That said, the initial block download is currently your biggest hurdle, as Bitcoin Core has to download the entire blockchain when launched for the first time, even with pruning enabled.
No, not Coinbase the company, but coinbase the special field in each Bitcoin block that is used as the sole input for coinbase transactions. A coinbase (or generation) transaction is the first transaction in each Bitcoin block, and it's always created by a miner. In our case, this means that ckpool.org pays its miners directly from each mined block, instead of paying its miners from a central wallet. In other words, the bitcoins that you earn from this pool come directly from each block that the pool successfully mines, and not from another party's wallet. There is no fee deducted from your payouts.
Web wallets (including Coinbase's) traditionally have not worked well with generation transactions, as web wallet operators have tended to abstract control of the underlying Bitcoin addresses from their users. A generation transaction to an address in your web wallet may therefore not be correctly credited to your account. (This is besides the fact that you have to trust the wallet operator completely to not mess with your funds, as they have full control of your wallet's private keys.)
Since you are unable to use Bitcoin Core as your hot wallet, I therefore recommend using Wasabi Wallet as your primary hot wallet. (Samourai Wallet is still in alpha.) It offers robust privacy-enhancing features and should work well with generation transactions. More users using Wasabi Wallet also means that its main privacy feature — Chaumian CoinJoin — works better and faster for all Wasabi Wallet users. You may then send bitcoins that you'd like to keep as savings to your Ledger wallet.
Bisq is a decentralized peer-to-peer exchange. It's like LocalBitcoins or Paxful, except that it's a desktop application that connects you directly to other Bisq users over Tor. Trading happens directly between parties, and so the time elapsed before your funds are credited to your bank account is dependent only on the time taken for a trade to complete (trade duration limits vary by payment method).
I do not recommend using Bisq's built-in wallet as your primary hot wallet, as it suffers from significant privacy issues.