The single external flash has its plusses, as do dual head flashes.
I don't want this to get overly long, but for a single head you have your pick from Nikon and from other brands. I do favor the cheap Chinese ones like those from Neewer. Naming a specific model is maybe not necessary since they are all similar, although you would want to weigh certain features like TTL (that being very common, really), or other special features that you think you might need. If a cheap flash fails you can buy 3 or 4 more before you start to catch up on the pricier models.
There are these smaller sized single head flashes like this:
https://www.amazon.com/MEIKE-MK320N-Speedlite-Mirrorless-Camera/dp/B01MU4BSOZ/ref=sr_1_5? or
https://www.amazon.com/Nikon-SB-300-Speedlight-Digital-Cameras/dp/B00ECGXAA0/ref=sr_1_3?dchild=1&keywords=Nikon+flash&qid=1616852981&s=electronics&sr=1-3dchild=1&keywords=Meike+flash&qid=1616852444&s=electronics&sr=1-5 or
https://www.amazon.com/Nikon-SB-400-Speedlight-Digital-Cameras/dp/B000KKPN5C/ref=sr_1_14?dchild=1&keywords=Nikon+flash&qid=1616852981&s=electronics&sr=1-14. , which are easier to mount on a bracket off of the hot shoe and move into a more forward position. A more forward flash of course means you can use at lower power and that saves on battery and recycle times. Its common to aim off the side, to give more interesting light and shadows, and you can hold a reflector off of the opposite side to help fill in shadows.
Then there are the dual head flashes. Nikon makes a marvelous one, though its pricey, and then there are these of course:
https://www.amazon.com/Meike-MK-MT24-Macro-Digital-Cameras/dp/B078WPMBCP/ref=sr_1_66?dchild=1&keywords=Nikon+flash&qid=1616853400&s=electronics&sr=1-66.
or the one I have:
https://www.amazon.com/laowa-vekx800-Flash-KV-800-Photography/dp/B01IYIYBTK/ref=sr_1_40?dchild=1&keywords=Twin+camera+flash&qid=1616853620&s=electronics&sr=1-40. That one is fully manual, so you will do all the work on figuring exposure.
And flashes like these:
https://www.amazon.com/MEKE-MK-14EXT-N-I-TTL-Macro-Assist/dp/B08BFHR8KL/ref=sr_1_1_sspa?dchild=1&keywords=Nikon+flash&qid=1616852981&s=electronics&sr=1-1-spons&psc=1&smid=A2XFVTH12VZC46&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFMVjJUSTVEN0lRRUcmZW5jcnlwdGVkSWQ9QTAyNTcwMTQzTVUyNVg2TkNPOEI0JmVuY3J5cHRlZEFkSWQ9QTA3NjM4NzYxVFhIWlk0S1JMVUc2JndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==.
There are lots of ring flashes like that, but this one allows for left and right power control, to add some shadows. Folks say that ring flashes are 'too good' in that they cancel out the shadows and make things flat. That is true enough. They will also readily make a ring-shaped highlight in eyes. Aggressive diffusion would help. Those who use these things will know from experience, whereas I have no experience with them.
But really the single most important consideration is diffusion. That is the biggest 'difference maker', and it can be vexing. You can try the usual commercial diffusers, and they are "Ok". But to really get it right, I can only recommend you join this Facebook group:
https://www.facebook.com/groups/1833312540247277/?notif_id=1541113934494651¬if_t=group_r2j_approved. and sort of lurk around in there. There are good simple ones to be found, but what really seems to do the trick most often are those big conical diffusers with a concave front face. The "Fisher King" design is very popular. It can be made for a big flash on the hot shoe, smaller flash in a forward position, and for dual head flashes.