The thing is I've never been in the mountains at night.

I think of 800 fill Aegismax Ultralight will do the job there?
https://wildproofgear.com/best-ultralight-sleeping-bag/ it is three times cheaper than WM Summerlite.

Everyone is so positive about Summerlite but it is really out of budget

I am not the expert in backpacking gear, but it seems to me this formula works for any gear best quality = highest price