I know both Forza Horizon 5 and Dirt Rally 2 work on SteamOS as I have played them on my steam deck

It’s hard to say which Horizon I would recommend. Horizon 4 is better than Horizon 5 imo (I much prefer the setting of 4 vs 5) but with Horizon 6 coming out in May it could be worth waiting for it.

Honestly check both 4 and 5 out and see which one looks more appealing as they play very similarly. 5 has more active players but if you just plan to play against AI then this is a non issue. Or go for whatever one is on sale if you just want to dip your toes in.

I will warn that Horizon games aren’t very well structured and can honestly be overwhelming with all the content added over the years (think joining an MMO with years of content releases).