Consider how much "countertop" space you'll have to set a glass or bottle if you put a 21" sink in a 30" space ... not much. Do get a deep sink and a faucet with a tall arc.

Would keep the new sink plain and not add "instant hot" or soap dispenser. You can get some attractive pump bottles for the soap dispenser and move it to left or right where it will be most convenient rather than have it fixed. An electric coffee pot will give you near instant hot.

In a configuration you describe, put sink to far right or left. Don’t center it. That will maximize usable counter top space. Be careful of faucet choice. High arch is good so you can get coffee carafes under and just general usage. Large swings will be problematic because it will swing beyond side edges of sink. So just think through how you will use and what clearances you might need.

You can do ANY under mount sink that fits in a thirty inch cabinet! You seem to have plenty of counter along side. It's a beverage area, not your main sink. :But yes, 21 inches is fine as you have it planned.

Simply ask yourself what you want out on the counter top all the time: Its a 78inch run with a 21 sink in the middle...... leaving about 28 inches of counter each side I have exactly same as I needed counter space in a galley kitchen. What do you need? A separate clean up /hand wash zone for fragile after a party? Yup...maybe.......: )

And yes, keep it centered - you're boxed in by the pantries. Space left and right is NICE.

If this actually is a beverage center, I really like the idea of an 18” sink on one side in one of the 24” cabinets cabinets. This would give you more than 54” of continous counterspace that you could use for plating or setting out appetizers and also have room for putting out ice and bottles of drinks on the counter.

A friend did the Elkay Mystic sink, mounted on the diagonal toward one side on her small beverage station. Looks great. Don't have a picture of it but here is a larger version.

You could do any kind of trough sink to save some space

My prep sink is 21” and it is centered in a 31” base cabinet. I think it looks fine. I could have fit a larger sink in that cabinet, but I wanted to maximize the counter space. I would have placed the sink a couple of inches to the left to gain even more counter space, but I wasn’t sure if it would bother me to have the faucet off-center.