The 10 should be about 5 months away, the question then is whether you can get hold of one. When I checked recently the iPad 9 had a longer waiting time for delivery than the Air 5, despite having been released 6 months earlier.
I would wait, but I would say that having been caught by the iPad 3 debacle. I bought the Mini 5 and 6 at launch, which I thought was a good strategy as there wouldn’t be something else coming along soon after purchase. Then again, I didn’t see the M1 8GB Air 5 coming.
Apple have released a new base iPad in September for the last 3 years, with no more than 18 months between updates going back to the 5th Gen. Whether they would hold off if the 9 continues to be backordered, who knows.