Two questions, if you have a thought for either, much appreciated!

The first is specifically for those who use breath in the nose as an object. I often run into difficulty getting onto the "smoothness" of concentration that is key to access conentration / jhana (which I've never experienced).

Partly, this may be because I try to make the physical sensation of breathing (i.e. the warmth and coolness) a smooth cycle, while the area where I notice coolness is slightly different than the area where I notice warmth, so the point of focus constantly shifts back and forth, instead of remaining steadfast. Also, there are times when the sensation itself seems barely noticeable and my attention breaks.

So the question is: do you follow the warmth/coolness, and shift the physical location of your attention to follow it? Or do you just stick to one specific location of the nose and focus on that, without being distracted by where the warmth/coolness would otherwise take you?

The biggest trip up is that I realize its more about the attention on the breath, rather than the breath itself. Any tips for keeping the attention constant?

Question #2: Does vipassana help concentration at all, if you haven't yet gotten 5 minutes of solid access concentration?

In order to develop concentration you need to focus on something that doesn't change. If you're going to use the breath the traditional advice is to pick a spot where you can feel the breath going in and out. Just below your nose or the upper lip can work. I focus on the slight sensation of the motion of air in and out of my nose, but you may need to pick something different. Here's what Daniel says about the basics of concentration meditation in his book:

https://www.mctb.org/mctb2/table-of-contents/part-i-the-fundamentals/3-concentration-the-second-training/

Here's a fairly traditional commentary on the Apanasati Sutta which is about breath meditation.

https://www.accesstoinsight.org/lib/authors/ariyadhamma/bl115.html

And here is the sutta itself.

https://www.accesstoinsight.org/tipitaka/mn/mn.118.than.html

You might notice in the notes at the bottom of the sutta as compared to the commentary that there's a disagreement about what is meant by 'breathing in and out experiencing the whole body' between the commentary and the translator. The commentary thinks it means the whole breath and the translator points to the actual Pali words used and says what's written means the body. I bring that to your attention not because I think one or the other will make or break your success but rather that there is disagreement. A good teacher with realization developed through the system they teach can be helpful for sorting these kinds of things out and  keeping you focused on what will advance your practice.

I don't know the answer to your second question. I came to vipassana as a technique after I developed access concentration.

Thanks for the great reply John.

So the object is the spot itself, and not the sensation of air that occurs on it, right? I think that's what's been tripping me up.

I have the tendency to follow wherever the sensations are the strongest, which ends up widening my spot until it's the entirety of the nostrils. How big is your spot?

I notice the pressure of the air on the edges of my nostrils. I think you do want to feel the breath just not in a different spot or different way on the inbreath and outbreath. We're approaching the limit of what I know which is pretty much about my own practice and what's in the sutta. The books MCTB by  Daniel Ingram, The Mind Illuminated by Culadasa and Right Concentration by Leigh Brasington are all good.

I've also found quite a bit of useful material on this website, awakenetwork.org  and reddit.com by searching them for jhana. In google that search for redditwould be "site:reddit.com jhana." The other two have search functions. Awakenetwork has Kenneth Folk's forum archives. There's a lot of good material there. Also look in the meditation journals on this website and that one. Some of that material is dynamite.

There's also a series of Youtube's with Nick and Kenneth Folk going through the jhanic arc. You may (or may not) be able to follow Nick along. If you can then you'll likely make faster progress although what they're doing is necessarily jhana lite. Otherwise they couldn't do the running commentary. Potentially valuable all the same.

Oh and there is a pdf copy of the Visuddhimagga on the internet which is very valuable if sometimes a bit opaque and very occasionally a bit off mostly in making it all harder than it actually is or by overstating the likely benefits of enlightenment.

https://www.accesstoinsight.org/lib/authors/nanamoli/PathofPurification2011.pdf

This is a treasure trove of resources! Checking out the books right away.
I've only read MCTB, and meaning to read the second version.
Thousand thanks.

Speaking of sticking to one spot, do you then stick to one spot of one foot for walking meditation?

(Previously I'd been trying to keep a constant flow of sensation heel to toe on left then heel to toe on right, but that's following something that changes, instead of sticking to one point)

I don't know. If you don't get an answer here then you might ask on reddit.com/r/streamentry which has a weekly question thread.