Is there a way to get all tags for an image via API with 1 request?

?page=dapi&s=tag&q=index&json=1&names=tag1+tag2+tag3+... has a limit of 517(?) and might fail in case URL string becomes too long for browser/server to pass around.

And what type of tag is Type="2"?

The limit on the tags API isn't a limit in our software, but rather a limit in the size that an HTTP request can be. There are similar limits to how big a search can be, or how big you can make your blacklist, etc.

Tag type 2 is a special secret. If you find one of the tags with that type, you'll get a prize.

>a limit in the size that an HTTP request can be
That's why I ask if there is a better way like to get a list of all image's tags data by specifying only the picture ID(s) in API request.

No, there is not. Tag type data is not stored in the post table, and the tags table doesn't keep track of what posts are in the tags (or even have an accurate count for it).