as someone who fell deep into the espresso hole over the past 8 years (from an $800 saeco to a $3000 profitec with a few in between), i see several problems with this.

First, CO2 -WILL- enter the brew. There is no way to stop it. This will create carbonic acid and elevate the pH. Which absolutely will impact the flavor. Part of the extraction "dialing in" process is determining the dose, temperature, grind, and duration for a new bean (and as it ages). The last thing you want it so add more acidity to the flavor. Some people do like sour espresso, I do not, and I think I'm in the majority.

Second, consistency of pressure. A CO2 canister output pressure changes with temperature and volume. An espresso machine uses a pump (and some a PID) to regulate this.

Third, this thing will probably explode after a few weeks. You can't subject plastics to this type of pressure. But OP did suggest using metal in the next version.

Fourth, health. Plastic + hot water is bad, add extreme pressure and it is even worse. Kick it up a notch using 3D printed plastic, which is a nowhere near food safe. I'm not too far into the "don't eat out of warm plastic" group, but i'm close. This would concern me.

I think it is a cool build. But a Bialeti is sufficient if you don't want to drop thousands on espresso machinery. Also in the running, the early Nespresso designs, not the rotary one. Both will get you into "americano" territory.