Uncrossing differential pair routing
Vias are quite big 1.3mm / 0.8mm hole, I can go down to 0.8mm/0.4mm
vias, is using as small a via as possible best practice for this sort
of issue? What's the technical reasoning behind it?

Ideally, for high speed balanced signals like USB you should introduce the same to one track as you do on the other. Here's what I mean (roughly): -

But, make your vias much smaller and therefore more likely to be closer to 45 Ω impedance on each. The impedance is determined by various things but, in short, a fat via pad will have an impedance that is too low and you might get some problems. Its impedance will be too low because the inductance to capacitance ratio will be smaller than that dictated by rules: -
$$Z_0 = \sqrt{\dfrac{L}{C}}$$
This site provides useful information about how you should go about doing this such as this for a couple of example of 50 Ω vias: -




So you have a couple of examples and given that they are 50 Ω, they are close enough to your required 45 Ω per track for this to be OK in my opinion.