my first two seasons were in steel and silver, then i hopped over to open last season and now i'm in IM.
i attempted my first open scrims when i was in silver, and i don't think we even won a single one until the very end of our silver playoffs (against a low open team at that). however, they also provided us with the absolute best feedback as to the efficiency of our strategies & DM abilities.
i'd honestly make the transition over to open as soon as possible, as you'll improve much quicker with esea than UGC. i committed just as much time into tf2 when i was in UGC, so it didn't feel like i had to commit more time.
also, the transition to open was much easier after i finished silver because i spent much more time MGEing than i ever did before. honestly, nailing the fundamentals of combat will allow you to tackle strategies much more efficiently than only trying to scrim. you can get away with doing so many things in UGC that are quickly rendered impossible against coordinated teams that understand and respect combat momentum and pressure.
basically, just put a hundred hours or so into MGE and you should be just fine to start tackling open. if you don't feel confident with your DM abilities, try it anyway so you can properly gauge your deficiencies.