Can I make a dynamic collection in Steam based on playtime?
No, Steam itself doesn't support that (yet). Although it says dynamic you can only filter by static attributes, such as state or genre. The Steam forums have quite a lot of feature requests for collections, but as of yet they remain unheard.
The best you can do is...
- create a filter for played and sort by hours played or
- install only games you actively play and filter by installed
For everything else you'd need a third-party tool, if something like this even exists.