Some general hints are:
Thermal vests and long johns as already suggested.
If you’re wearing trousers, a pair of tights under them, and a pair of socks over the tights.
Any extra layers will add warmth - for instance a plain shirt under a fancier one will be warmer and also looks ace if it peeps out at neck and cuffs.
More specifically for Dawn, leaning femme because that’s what I go for:
A hood with a shoulder cape keeps your neck and upper chest warm (and if woollen, dry as well) which makes a surprising amount of difference. Dagging (cuts in decorative shapes) along the edge of the cape is super Dawnish. Look for reenactment merchants, or see here for a tutorial to do it yourself:
https://www.virtue.to/articles/hoodlum.html
In the past I’ve worn a short-sleeved fitted kirtle in a lightweight wool, with pin-on sleeves for extra warmth in the evenings. That, with a cotton smock and leggings underneath plus my caped hood has been perfectly cosy for dry autumn events.
For a something a bit more dynamic off the shelf, the Burgschneider Ranwen dress ( Undergarment Ranwen - Grey ) has nice flowing lines without the extra fuss of handling a full skirt.
This pattern from Simplicity has two lengths/styles of gownwhich could work very well for a combat + a casual set.
Houppelandes are really cosy, luxurious medieval overgowns with hanging sleeves. The female version traditionally was long enough to pool on the floor (not ideal in a field), and belted just below the bust, while the male version was often just to the knee and belted at the waist. Either looks good on any gender and makes for a nice Dawnish ‘coat’.
If you have any thoughts about whether you want to go masc or femme, full length dress or not, off the shelf or make your own, do say and I’ll try to tailor advice to be a bit more specific!