It's been quite some time and in the meantime I glued together neck and body to let it cure during the Christmas break. Before I did that I checked the neck angle several times by putting one or two strings and checking their height. I then adjusted my shim made out of maple veneer until I liked the result.

After almost a month I sanded the oil finish moooore or less flat, ordered a new bottle of tru oil and started oiling again.
I changed my method of application and am now using a cut out of the soft side of a kitchen sponge and I'm oiling the guitar hanging in the closet (with windows open, gloves and mask, just to be sure).

This time around I'm keeping track how many times I put oil and am currently counting 8 additional times with one light sand in between and I think now the finish is slowly becoming what I want it to be.
Thanks for the advice, I will keep you posted and once I'm done send additional pictures