@Charles Tottington Well, I did say "warbands" which should have tipped you off that it was smaller in scale (like most SciFi wargame😆).
“Games like W40k and Xeno Rampent?” pretty much anything that says “platoon level Sci-Fi” and has vehicles on the front cover. These are the ones I know about for sure that work with 28-32mm minis without hassel:
Shock Force second edition, by DemonBlade games would fit the bill. This is probably the one I am going to check out with the Damned in mind since it is a vanilla post apoc themed platoon level fast play wargame that has been tested out pretty well and has a unite creation system. The one issue it might have is that I heard the vehicle rules and option where somewhat limited, though it was supposedly not hard to homebrew that up with Shock Force and it might have some synergy with things like Crank Engine from the “one page rule Jam.”
Star Grunt II, a vanilla SciFi paltoon level skirmish game originally meant to sell 15mm and 25mm scale Hammer Slammers style hover vehicles (which you can still buy from Ground Zero Games). Its complex and a touch more realistic, but there is a recommend “simple play” in the core book and even with hard mod on it’s probably still easier to play than 40k. Also its free digitally and has a relatively lively supporting community on Facebook.
Boltaction which has similar rule mechanics as 40k, has fan made errata for SciFi that should cover most things (though you may want to go with making the assault rifles and automatic rifles as your main infantry guns) and rules wise likely works great with WA’s Death Fields models (frankly the Bulldogs look almost perfect for Boltaction).
onepage rule’s grimdark which is essentially 40k for those not wanting to read brick length books.
Antares 2.00 is supposedly more setting neutral than version 1 so one can play other settings with it, and its free.
Beyond that there is probably 5 to 10 other ones for 28mm SF on Wargame Vault.
You could also in theory modify platoon games meant for 15mm or smaller minis so they work for 28mm. There are also a couple of vehicles centric games out there that are in theory scale neuteral or modifiable (Gaslands, Mekaton,etc.) that might be worth looking at if you plan on scratchbuilding a large motorpool and/or went heavy on the trucks (or have some large giant robot models sitting around you want to add/fight your Damned army😆).