Daily League Sudoku #36: Sum Relations Sudoku

I have seen puzzles similar to this before, except in a Killer Sudoku format. In those puzzles the relations are given between different cages. Here the relation is between the sums of adjacent cells on either side of the symbol.
This is again a harder puzzle. I don't know exactly how hard it is, but there's definitely no progress to be made if you don't understand the opening. I wasn't expecting to be able to make this variant without any givens. But I managed to create an opening that works without any givens. I think it's a nice and interesting opening. The middle is probably a bit easier, but I think the end is again a little bit harder, but worked out nicely in my opinion. I'll think about making something easier again next week, but I can't exactly promise that. When I find an interesting opening, I just run with it. And interesting is generally hard. Enjoy this puzzle though.

Rules for Sudoku

In this Sudoku, the symbols indicate the relation between the sums of the 2 cells on either side of the symbol.