When does the mainnet version of this DEX roll out? It seems interesting.

Is the max supply capped, or do the devs have any plans to change it later?
The supply is capped at 300 mio. Fees on the DEX will be used to buy-back HDN, and burn them off the total supply. So depending on volume, the supply will be deflationary.
The DEX is ready when it's ready, it has been stated many times on their Discord, that they dont want to release anything that doesn't work like 99,9% of the time. Should be ready by Q3 at the latest I believe.