My Blackstone griddle has been absolutely amazing over the past 2 years. I usually use it a couple of time each week and it has held up really well. After cooking, I scrape what I can, then squirt water and scrape off the rest. Squirt a little more water and wipe clean with paper towel. Then squirt some veg oil and wipe with paper towel.

I just started to get some spots where the season is flaking off. I first noticed it when I found some rust spots after about a week of no use. Now, the spots get a little bigger after each use.

So, I assume it's time to start over. But what's the process for this? I cook with cast iron a bit, but I've never had to deal with this. What is the best way to re-season a flattop griddle?

I can't speak specifically to the blackstone surface but if you need to start over, something like this kit - https://www.amazon.com/3M-COMMERCIAL-461-08297-GRIDDLE-HOLDERW-Kitchen/dp/B00201EFK8/ref=sr_1_5?keywords=3m+griddle+screen&qid=1640605780&sr=8-5

The screen is for heavy duty, i.e. - oops, i burned sugar on the surface and regular maintenance won't get rid of it.

once you've hit the surface w/the screen (use oil still to lubricate) and throughly cleaned w/paper towels and oil you can reseason.

I didn't know you could "season" stainless steel?

A Blackstone is not stainless steel. It is cold-rolled steel. If the seasoning is flaking off, as with cast iron seasoning, the initial seasoning did not adhere properly or the seasoning is worn off (cooking acidic foods, cleaning with acidic stuff, cooking at too high a temperature for too long). Very high heat can release seasoning.

Would suggest to reason with a few rounds of oil and/or cooking some bacon. It sounds like you're doing the right by ending every cook applying oil at the end.

Blackstone has a video on that where their guy says they need it after a couple of years or maybe more of cooking. It's seasoning and even some carbon build up.
I bought a rotary sander, 60 grit paper and hit it all over. It takes some time but for my 28" I was done in an hour or two at the most. You will go thru quite a few pieces of the sand paper. Take it down to bare metal, work the corners by hand or a scraper. Rinse off with water and turn on heat to dry. Begin your reseasoning w oil process. After that The blackstone guy just tossed two lbs of bacon on it and cooked all over the griddle and said it is ready to go. Worked for me and I did it on 3 different griddles this past year.

Edit to add the YouTube video I watched was by Todd Toven who travels around for blackstone and cooks. He is kind of annoying but has some good tips.