I've done a video for my 100, 666 (lol) 1000, 2000? 5000 (i think?) and stopped after 10,000 I think.
up until 2-5k the videos were basically self indulgent videos that everyone does when they're super hyped to get that number.
"luckily" for me, 1000 wasn't a requirement for anything back then, so I didn't "need" to hit 1k, all it was, was a number, and a fun number to hit. Gave me an excuse to make a celebration artwork, but other than that it was all self indulgent and otherwise meaningless. the 666 was fun lol, I just wanted to hype up that number because I'm still into metal music so I found some humor in the number
I think at 10,000 I changed tone. Instead of thanking people (which let's face it, no one other than your core fans care about) I focused more on sharing art submitted to me by my fans.
Those videos were time consuming to produce, but I think it was more rewarding because it celebrated the impact I made through seeing everyone else's art based on my drawings. Way more impactful with a milder level of ego and self indulgence. It more celebrated the community.
I think from here on I might use milestones as an "excuse" to give more shoutouts. And when I say shoutouts, I mean giving some promotion to a smaller art channel that doesn't ask, and I just notice them and feel they need more exposure.
At the end of the day, if the celebration serves to help others as well, then go for it. If it's self indulgent thank you's, you can do one for 100 subs and 1,000 subs but otherwise I wouldn't expect much from them. But just because they might not perform doesn't mean you SHOULDN'T make videos on those milestones