We have had our 30mm solid Douglas fir floorboards in 24cm width here for a few weeks now. If you want solid wood, then you know that it can sometimes creak or a joint may vary depending on the season. In some places, in my opinion, we also tightened the boards too much to avoid gaps, which of course was nonsense. However, we really like the floor as it is now installed, another person might not like it.
Since I also had solid wood floorboards in my former house, I consider the usual wood characteristics to be a desired normality. I would advise anyone interested in a wood floor who prefers a gapless floor not to install solid wood boards in their home. Every material has its peculiarities, even though nowadays hybrid materials are often used (I hope this term applies here), by choosing tiles, vinyl, etc. in wood look or considering a floor with roughly 0.1 millimeters of applied wood dust to already be called a wood floor, thereby simply erasing the typical material characteristics. For me, that is unimaginable.
Without knowing your project in more detail, I would definitely not want to use an adhesive; with wood on wood, I see no necessity for that.
Depending on the height/condition of the subfloor, for example, I would screw your boards onto floating battens, which is how we did it here, with loose insulation material between them as desired. We screwed into the tongue and pre-drilled each time, which is why the boards did not crack.
What I don’t quite understand about your offer is why the subfloor should be sanded at great expense; this could be more easily compensated for with battens. Apparently you have room upwards, so battens would be my first choice.
We bought our boards before the so-called wood crisis on an online classifieds site for about €40 from a great carpenter who had a larger quantity in stock. If you watch the market a bit and are, above all, somewhat more open with regard to the wood species, you could definitely find something interesting; oak boards are certainly expensive, but I wouldn’t need them. We chose Douglas fir because of availability/price/width/great seller and took a chance on a color change, which ultimately worked out, although we were quite worried about it; Douglas fir tends to turn reddish, which we would not have liked. But a pigmented floor oil made it possible that it didn’t.
I wouldn’t pay the €90 for the offer and wouldn’t have the floor installed by this craftsman. I always pay my dues correctly but currently often experience that others like to and quickly reach into my wallet because builders must probably be glad that anyone shows up at all these days. In my opinion, such an offer is free, but the craftsman has not understood what is important to you and that will probably show in the further execution as well.
I think you can find a craftsman who will lay such boards on the floor, it is ultimately no rocket science for a craftsman, maybe your yet-to-be-found wood supplier might also have an idea.
From my point of view, I would encourage everyone to put such a “real” wooden floor in their home, because for me a 4mm wood overlay would feel more like veneer; as I said, that may just be my quirk, I prefer either/or.
In the utility room, we installed a 30mm spruce floor from our wood supplier, honestly I also like it, great quality and the price is between €25-40 per sqm depending on quality/supplier. I cannot see any added value for my purposes for €150 for another type of wood.
€100 for installation per sqm is in my opinion totally unreasonable, I would rather pay by the hour. I believe you will find the right craftsman once you have found the right supplier; ask a sawmill near you or look online. What I have read from you suggests that the customer and craftsman do not fit together here!