Stardust Limit?

I've had over 1,200,000 and know nothing of any limit

Nope. I have more than 3M stardust.

I met some people in NYC that had 5 million stardust so I dont think there is a limit to dust.

There probably is, but hopefully it is so high that no one is expected to reach it.

Most normal programs have a an upper limit of how high certain numbers may be. Occasionally it is too low so under certain conditions it may give unwanted consequences.

4m, just fine, me rarely use stardust, coz bit vigilant about rebalance ;)
Invested lots dust on lapras and ex-gutor and others long time ago then they were nerfed, its just investing in vain, so better save it for future fixed meta ?

There are people approaching 10 million stardust. You are fine.