When you say budget I'm not sure what value that is but the Suntour XCR Air is a good entry level air fork with available spares. If you just want to go a little beyond a gravel bike you could consider a rigid fork. Assuming your bike can take some pretty big tyres, bigger than typical gravel bike tyres that might be enough. You can get 27.5" plus tyres which are sort of half-way house between normal mountain bike tyres and fat bike tyres. Then you have no fork maintenance to worry about and no real difference regard weight penalties. To be honest when I'm looking for forks I tend to see what is available at a great price,
ebay, gumtree, facebook marketplace and then find out how good those forks are once I know the model. It really depends on if you do many drops or jumps etc. The big tyres can offer some amazing grip over rough ground but not so good if you will be getting any air between your bike and the ground. Also speed; suspension can adapt fast over varying terrain if you are going fast but big tyres are more about tackling rough ground at leisure speeds. I'm fairly biased against suspension but that is because of the sort of riding I do. I don't want anything that will cause issues on longer rides.
