The phoenix is a dedicated 3/4 length under quilt (52" long) and won't be suitable as a top quilt at all. Which thermarest do you have and what are its dimensions? Most are only 20" wide and that will be a challenge to keep under you. Also only being 3/4 length, you're going to likely need some extra insulation for your lower legs and feet.
If you can swing it, I would save up for a dedicated underquilt as it will allow for a great degree of comfort in a wider variety of conditions.
Cheers
Brian
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