I've got the soda stream and I drink about 5 liters a day. They have a subscription plan and you can exchange your cylinders. That's kinda pricey, still cheaper than buying packs of seltzer long term though.
I have a few cylinders still full of gas, but I already purchased this adapter thing for them to refill cylinders myself and will be buying beverage grade gas from a welding company near me. That will make it exponentially cheaper.
I don't typically use flavors, so no recommendations there. But yeah I like this thing a lot tbh