Problem with such textures is the fractal-like nature=the more you zoom in the more details you (are supposed to) get.
Inkscape's svg -scalable vector graphics- and any other vector format means a theoretically infinite zoom range.
That cannot be followed with the infinite details. At some point you'll max out the rendering/processing resources so you'll have to stick with a direct output source.
Additionally, by vectorising a raster image you won't get any "unseen" details.
As a result a vector substitute theoretically can look sharp only in a smaller resolution than the original raster image.
Thus in my humble opinion it's a bit pointless to vectorise a paint blot texture. If it doesn't need to be explicitely vector content. Can hardly imagine someone cutting out each layer of the trace with a vinyl cutter.
The other suggested route was to use a filter substitute. Although svg filters are generated only on the rendering level, they are live and theoretically they can be exported in a higher resolution -can generate a closeup blot texture. Also resource heavily though.
tl;dr
multiple scans can give you a half decent result. Inkscape uses potrace which as far as I know captures the whole image in one, splits up the lightness channel and traces every tone value as a single scan,
then uses a process to average the colour of the given tone and uses that hue/saturation value on the output paths.
Which then are put one atop another.
Side effect that there is a 256 scan number limit (as far as I know). Makes sence if thinking of that covers the full lightness range available in the colour model used.
As an artifact edges can be bit washed out at otherwise sharp lightness transitions and hue can be visibly off.
Also the rendering gap issue can be prevalent -seeing through between separate path edges next to eachother.
A cheap trick to handle that is running two scans.
First, run one with multiple scans in colour mode at a lower number of scans, then put another scan above it with a higher number.
The corresponding part in the manual is
here.
I think you are on a right route with this already -the 256 colour limit and the approximate colours etc. are bummers.
Illustrator uses a different kind of algorithm for tracing which processes the image in smaller bits at a time.
Like filling a small area with a bucket fill&dropper tool, moving next to the traced area and repeating the process.
That still doesn't solve the rendering gap issue though in my humble opinion the result looks way more realistic.
Sorry for not being more helpful with the problem.
Will try to make another watercolour drop filter later of that attached image.
