The difference between a dinghy and a heavy keelboat, is that everything happens more slowly, but has more force behind it. A heavy keel boat is less sensitive to every little puff and shift that would require instant correction on a dinghy.
Generally, if you learn to sail on a dinghy, it'll help you be a better keelboat sailor.
I started with a 25' boat, and upgraded to 30 feet. The difference in handling and difficulty, was not so great that you should feel the need to do this. I upgraded sooner than I expected, because I wanted a boat with better performance, not because I was worried about handling a 30 footer.
Learn to sail on a dinghy, bum a few rides on keelboats to get used to the difference, then go out and buy your 30 footer. You'll be ok.
S/V Old Shoes
1973 Pearson 30 #255