Here's (https://www.sciencedirect.com/science/article/pii/S1296207421000546) a curio. A digital reconstruction of equipment not for illustration but to carry out virtual testing.
Absolutely fascinating and right up my straet
Very interesting to see.
I did it the easy way: wearing Thorsbjerg style tunic, trousers, and mail shirt.
A belt definitely helps when wearing mail. Mail does not inhibit movement very much- in my younger days, I could run when fully armed with mail shirt, helm, shield, spear and sword.
Underpadding is not really clear. many early nedieval re-enactment groups use fabric armour, but I'm a bit dubious: there do not seem to be any references to it, or words for it, in Anglo-Saxon or Norse sources.
I'm with Peter with regards the belt. Indeed having tried it, I would have thought a belt virtually essential as it puts some of the weight on the hips rather than all hanging down on the shoulders