Like /u/boudewijnrempt said, the correct answer is 'don't', because it's just not going to give you good results compared to a tablet, any tablet.

Stabilizers can help a lot or lots of experience can help a lot, but it doesn't really save the issue with mouse.

Until Affinity and Inkscape gets good gradient mesh tools, Krita seem to be the best choice for 2D rendering on my end. For me, I don't need to use a tablet to generate beautiful looking 2D renderings of products in Krita, but I'm not a typical Krita user, nor I am Krita target market, but it works for me, so. If he's using vector tool in Krita, I don't see why he should use Inkscape or Designer when brushing using vectors in Krita can help in rendering smooth objects unless he needs vectors, but in most use case, that's not going to help most Krita users. I am actually using Inkscape as my vector content creation main software, and Designer as a publishing software because Scribus is terrible for now. There's no offset function, that's a first sign that Affinity needs a lot of polishing.

Digital art on krita