Quilt or Bag liner to increase bag temp.

Forget the Thermolite (or any other liner) to give you anywhere near a 10-15F boost!

Ignore the STS marketing hype; the Thermolite is good for 3-5F temp boost — at the most. Other lighter weight silk or coolmax lines will give you about 2-3F boost. Basically, liners are primarily good for keeping your bag cleaner — and only secondarily as a minor temp booster.

If your 30F bag is spacious enough that you can insert a quilt or another bag without compressing the insulation of either — then you may be able to get a 10-15F boost — thus allowing you to use your 30F bag (assuming it's rated accurately for you) down to 15F or so.

Liners will give you like 5 degrees at best lol

No way you will get 20-25 degrees BUT…. I have taken my UL-20 quilt, STS liner and a bivy down below zero and stayed surprisingly warm. I was wearing capilene 3 uppers and bottoms, lightweight hiking pants and a fleece pullover and warm hat. I actually think the bivy may be the better bang for the ounce.