Upvote:3
A Bewijs van garantstelling en/of particuliere logiesverstrekking would be the most logical choice. It would establish that you have a bona fide invitation and that your sponsor is serious about hosting you.
Note that, from the perspective of the consulate, the situation raises many questions and I would not assume that the genuineness of the relationship is the only issue (Schengen refusals are quite broad and only use standard formulas so I am not sure how you drew this conclusion). But the Bewijs van garantstelling is in any case a good thing to have for this types of visit.