Betsy good day.
So that FA Amplify 1500 is pretty much entry level kit. Summer to Autumn, at best. As @PhilipV says, not very warm. Cheapish fare.
And if it’s not new, as in older than 3yrs, the synthetic insulation has lost considerable quality already. I have several comparable gear items and can tell you synthetic starts losing insulating power much much quicker than down.
Your sleeping pad can preserve a lot of warmth. The earth will suck you cold. Ask your Killi tour operator what pad they will be supplying and what it’s R rating is. You want R3 and above. You want a name and information, not “don’t worry we’ll make it nice for you”. You can buy a little extra sheet of closed cell EVA foam (Sondor supplies in Gauteng) or a cheaper pad from CUM or Outdoor Warehouse (OW)and chop in half and lay beneath pad provided for thighs and torso.
Not all thermals are created equal. Merino is yr best option with a price to match. At least you can use them in the rest of your life too. Look at Icebreaker at Drifters or OW. They also don’t stink like those plastic thermals. Bonus after 2days.
For good warmth to weight ratio you can also look at First Ascent K2 Powerstretch items. Very toasty and light. Again,good for general use.
On NH sleeping bags I cannot say much. On paper looks good but the ULG700 is expensive and at that price you can do better. I’ve gone to the supplier in Pta had a look and what I can say is the sleeping bags stink, unwashed down. The fabrics looked cheap too. Ultra light does not necessarily mean high tech. So I judged them… (edit: which might be unfair)
Here are two other bags to consider. Though whether they’ll cut it on Killi I don’t know
Of the two I’d say the marwees has better construction and should thus be warmer.
You should be able to resell either quite easily.
But I’d look at improving on clothes too. This is not an expense you’ll have to recoup later. Sleeping bags in SA are generally lower quality than international fare.