It is a tough one and no mistake.
First off, temperature ratings. I hadn't looked at the specifics for Snugpak (recently, I have had two of their bags, a No6 and No12 Osprey, which was rated similar to yours), but here is their statement bearing out what I said about tent, but not a buddy.
https://help.snugpak.com/index.php/knowledge-base/temperature-ratings-for-sleeping-bags/
"Snugpak’s performance figures are drawn from over forty years of experience and customer feedback. Our comfort figure assumes you will be sleeping lightly clothed (thermal base layer or PJ’s) within a tent on a good quality sleep mat, at this external temperature the majority of users should feel comfortable and get a good night’s sleep."
I have used a fleece liner when camping in October, and it did boost the comfort of my bag, but it was a very bulky solution. I have a silk liner, and while it adds a little warmth, it isn't the kind of difference you will be looking for.
You do lose a lot of heat to the ground. I have an early NeoAir and its insulation value is rather low (R2.5). Check your pad's R-Value.
https://www.thermarest.com/blog/r-value-meaning/
https://sectionhiker.com/sleeping-pad-r-values/
Hydration is a fine balance act, too much and you have to get up in the night, too little and your core temp can actually drop.
Last meal of the evening. Did you get any fat? You metabolise fat more slowly, think of it like a low, slow, burning fuel to help stay warm through the night.
Cutting out air movement over and around you, I know you want the open plan of a tarp, but maybe a little less cross ventilation would help without cutting you off completely. In my hammock, I am always amazed how much temperature difference just having the midge netting zipped up makes. Perhaps peg one long side of the tarp to the ground.
Picking your bed room can help too. Evergreen top cover can help on a clear night, fallen leaves and needles under you is better than wet soil. Not camping in the lowest point around might keep you above the pooling coldest air. Cold air flows like water.
Finally, do you have a summer weight sleeping bag? I have doubled up my main quilt with my hammock underquilt on occasions, when sleeping on the ground. Likewise used the quilt over top of my sleeping bag in the winter in Canada. Still wasn't all that warm, but better.
A blanket in the bag could help. I would advise against wearing more than base layer clothing on your lower body since my experience has been that wearing lower body insulation is counter productive, like the difference between gloves and mittens.
Going to bed warm-ish might not have been enough. I know of some people who do a few press-ups before getting into their bags. Not enough to sweat, but enough to get their internal furnace working so they warm up their insulation system with "excess" heat.
Not much else to suggest. It all comes down to the simple principles of starting with heat and not losing it to conduction or convection.
Good luck, and good on you for getting out at this end of the year!
Chris