I think it’s just down to timing. This weekend I queued for Polaris and ran it two times in a row.
On the second run my teammates died when the last boss had half his health left, and I solo’d the rest of the fight (hammer/sword), so you should be good for soloing Polaris.
I’ve definitely soloed it at level 20 with an alt (blood/AR). It helps if you have a healing ability.
edit: and I encourage you to solo the dungeons once, just to check out and appreciate the locations (and get all the lore). Although I don’t think you can solo the Transylvania dungeons.