Once you have a CO2 tank and regulator the only thing the sodastream is providing is the interface from tank to water bottle. Direct into the bottle is the way

Once you have a CO2 tank and regulator the only thing the sodastream is providing is the interface from tank to water bottle. Direct into the bottle is the way. You can buy carbonator caps for different bottles from a homebrew store. This is a routine maneuver for homebrewers and the risks are small & well understood if you approach it using their gear, knowledge, and technique. Once your current sodastream bottles expire you can replace them with something else entirely. Getting the tank and regulator is like 80% of breaking free of their stuff entirely, just finish the process.