Long story short
This novel gets less and less interesting - better forget the last few volumes entirely
Volume XX was about a "car race" in another world
Volume 19 was entirely about the events that where previously explained how they would unfold
Volume 20 is about his harem weddings
If i try thinking back to when the story was progressing in an interesting way it was probably the first 3 to 5 volumes. After this it just gets more ridiculous - more harem, more harem, some old pseudo android women/girl appears, etc
If you cross out all this, it's still pretty interesting but the story isn't worth the 20 volumes; at max about 8 volumes without harem filler.