Absolutely second the recommendations of Doom and Quake here in the thread. Boomer shooters in general. Even if the movement can be really fast, playing them on your own can be extremely cozy. Just get into the rhythm of circle strafing, shooting and weaving in and out of cover and you’ll be in the zone very quickly. Bonus point, that both Doom and Quake have 30 years of EXCELLENT quality player created content that can keep you playing fresh new levels for as long as you want to. You could play them for the rest of your life, at your own pace and preferred difficulty.
The new rereleases of both games even bundle a mod browser that you can access with zero knowledge of modding, just hop on.
This mod is awesome. It can convert any fully vanilla assets map pack into Doom 64 too. And there is a project that recreated Doom 64 maps in Doom 2 format. Yes you can stack them!