You can effectively thicken it by stacking another on top of it. Go to Engrave/Music Symbols then Category: Articulations. Select the tenuto (above or below, you’ll have to do both), then add another tenuto. A major problem encountered here is that Dorico’s editing window in this instance (and in chord symbols) is very buggy and is not WYSIWYG, so you have pay very close attention to the Attachments tab. If you just position it visually without changing the Attachment setting, it will look correct in the window, but not in the score. Select your new additional tenuto, make sure the attachment is set to “from: Top Left” and “to: Baseline Left.” In the Component tab, the X offset should be 0 and the Y to however much you want to thicken it. Here I used 0.5 and it is quite thick, but obviously a smaller value would thicken it more subtly:
