Ok, somehow I've managed to do this on my own.
I'm just gonna write step of instructions for the community.
1. Document ---> Resize Document...
Change from current DPI to 400
Make sure Affinity has adjusted the resolution according to the increase in DPI
Resample: Lanczos 3 Non-Separable
2. Layer ---> New Adjustment Layer ---> Levels Adjustment Layer
White Level around 93%
Black level stays changed
3. Layer ---> New Live Filter Layer ---> Denoise Filter
Everything maxed out without enabling extreme option.
Is this the best method? Most certainly not. I'm only a beginner in this image editing software debacle just so you know.
Have a lot of details been lost during this process? Hell yes! But you be the judge Image. I can't upload an image for now on this forum for some reason.
I don't guarantee that this method works for all images nor that it is the most reliable and quality preservation technique out there.
Edit: But what I could say is that if you would repeat the whole process on the image again and downsize the image to your original intent I guess you should end up with a better quality image overall.