There's a learning curve that sorta works the same regardless of what age you come in at, so I don't expect him to continue improving up until his early 30s, at least not in anything but fine detail.
First 2-3 years they are spent getting up to your ultimate speed ceiling, then by year 4-5 they have reached their ultimate pace and then it's about piling on the experience and tweaking your approach to find marginal gains. Verstappen is already the finished article in terms of speed, now he needs the experience of fighting for a championship rather than just the odd individual race win that will shape how he goes about his racing in the future. You can't drive the same when your ultimate aim is the championship compared to when your aim is just to win the race.
But you are right, Verstappen is legit. Anyone who's watched F1 down the years can recognise that he is clearly very very talented and becoming a formidable all round driver. It has to be said too, that I think Leclerc may be just as good, but with a few years less experience than Verstappen currently.
This is where mentality comes into play. In terms of pace and ability to race (e.g. overtake), he is for me easily in the top two and has been for a while now.
He is becoming a more rounded driver in term of race management, which utilises a mixture of composure, resilience and guile with the usual confidence et al.
Yet, I still have some doubts regarding these elements because I still get the aura of rashness, where he might not be fully versed in how to control himself.
With Leclerc, he's far from perfect either and has been prone to mistakes, but I think long-term he may have more composure than Max if not naturally as fast.
This is all my opinion though, so people may disagree and rightly so. Max has calmed down a fair bit in recent years, so he may be able to resolve it.