Regarding the actual question, namely whether the architect's cost estimate is reasonable, unfortunately no one will be able to make a well-founded statement from a distance and without knowing all the details about the building.
Friends of ours also renovated a listed half-timbered house and one cost driver was indeed a very "picky" monument protection office. For example, old beams had to be replaced entirely with historic oak wood, which was really expensive in the required length, thickness, and quantity. Visually, new beams could probably have been treated in such a way that essentially no difference would have been visible, but that was not accepted. I am not familiar with half-timbered houses and do not know if this is always the case, but at least our friends' architect had not taken this adequately into account in the cost estimate. However, your financial framework sounds as if a few additional costs could still be manageable.
I wonder whether with a 3000 sqm plot there might possibly be the option to divide it and sell one half as a building plot? If that were an option, you would have a great buffer for unforeseen budget overruns or other expensive wishes, which might alleviate some worries. One could wait to see if it is necessary at all and then decide whether to live somewhat more frugally or to have a smaller garden.