The Flicka is a great design, and 6.0 kts is entirely believable under the right conditions. That would be her displacement hull speed.
A larger boat may be more comfortable and faster, but would certainly be more work to operate and more costly to maintain.
What Sven seems to be aiming for is the absolute minimum needed to safely go to sea in.
Since a smaller vessel is going to require a longer passage, and that passage is going to require even more stores, a much larger portion of that vessel's displacement is going to have to be made up of payload.
This is going to be a major design constraint. The amount of ballast and how it is to be deployed is going to be limited too. A drop keel may seem like a good idea, but it would have to be in the widest part of the hull, the very place the skipper is likely to be living. And he or she cannot be shoved forward or aft, if the boat is to have the proper trim.
So, the ballast must go under the boat, or at least under the skipper. And the sailing draft should be kept within reason. Keel strikes seem to ruin every bodies day. A lot of voyages I have read about have ended or have been severely delayed due to keel strikes.
A shallower draft per size heightens the access of the tiny boat, which is one of its few advantages.
So, shallow sailing draft must go with limited ballast. And this must come with very high ultimate stability.
Such will effectively limit the spar heights and weights. And this will, in turn limit the size of the sail plan.
My Lola 520 design, for example, has a sailing draft of only 1.5 ft, yet has a positive righting moment at up to 140 degrees of heel. It can carry 1,000 lbs of stores or more for a total displacement of 2800 lbs.
The sailing rig has 140 to 160 sf of sail for an S/D of about 11.0 to 12.0, which is more than twice that of Sven's creation.
I think Sven skimps too much on sail area.
But, like a mountain bike for the road (to make the actual road trip on), the mountain bike of the sea may be more suited for younger adventurous types, and really may have no place now that older, but mostly sound, larger larger boats are available for often next to nothing. They can be put right, if the owner is willing to do the work, for a fraction of the cost of building one of these mini-voyagers.
But I like playing with design ideas for the type.
I consider an ocean capable sailboat with a sum of 30 ft (in Beam, Length an Draft) and a loaded displacement of 1.5 short tons or less, to be a mini-voyager.