I assume that the temperatures given are Celsius, in which case the bag would be more than warm enough for 3 season use in temperate climates. Keep in mind that temperature ratings are not standardized across the outdoor industry, they are just the manufacturer's estimate of what temperature a bag is comfortable at. As you approach the lower end of a bag's comfort rating, factors like the size of your tent, what and how much you ate for dinner, your own metabolism and whether or not you are wearing a hat will all affect the perceived warmth of a sleeping bag. As a general rule, temperature ratings on cheap bags tend to be more optimistic than temperature ratings on better quality bags.

A fleece liner might add a little warmth, but only if you could fit it in the bag without compressing it or the bag too much. Some kind of overbag that is big enough to cover the bag without squashing it might be more effective, especially if it was somewhat windproof. A silk liner would also add warmth.

Lots of people think that sleeping bags are warmer if you don't wear clothes, or only wear a single layer of good long underwear. I've found this to be mostly true, although a good wool hat or balaclava always seems to help.

Which Thermarest are you getting? You could use a thicker model in winter, or combine the thin one with a cheap blue foam pad for winter use. The reflective ground sheet sounds like a good idea, too.