Drinking milk, even in small doses is gonna have a little effect on insulin levels, which will slow your progress.
However, if it’s something that helps you and it extends the amount of weeks or months you keep to a 18:6, it’s better for you in the long run.
You can also move your 6 hours around so that you can still enjoy the creamer and then maybe slowly wean yourself off it and drink tea plain.
It depends on how much of a fasted state you want your body in. Anything other than black coffee, water, and herbal/black tea (so tea with nothing added) is going to raise your insulin and blood sugar and you will not be using the fat stores in your body as vigorously. That’s just the science.
I have seen people recommended plain sparking/carbonated water - and that is a good one, though obviously that’s not a hot drink.
I personally do black coffee upon waking, then water throughout my fasting window. During my eating window I add an electrolyte mix into my water which I like the taste of and that helps me meet my 80oz water goal daily without breaking my fast.
Ok try and give ground black coffee a go. I love it now and wasn’t sure at the start. If it seems alot of hassle get a one cup coffee maker. So handy and easy to clean ☺️I would forego milk anything during fasting times, give yourself that as a treat in your eating window