i would reccomend something that has an ingame timer for you first run, like micro platformer, or celeste or any game really that has an ingame timer so you dont havo to set up your own timer.

Hello! Enter The Gungeon were kinda steep for a first timer considering that you need to understand the movement and dungeon mapping of it. But, if you did enjoy EtG casually (or enjoying the runs of it), consider taking the shorter category first before getting the long ones.

As always, my suggestion would be any game that you play casually, so that you understand the basic of the game yet ease out by the speedrun aspect of it. Other than that, you can try any webgames / Minecraft (& Roblox) short category / any RPGMaker games as a go to.

Hope your having fun finding your first speedrun game! :)