About the only way I can think of to lower your GH would be to use RO water to some extent. You could dilute your tap water with it or use it entirely and remineralize it. I don't think you're all in one fert is contributing to your GH very much. While it may contain some calcium and magnesium it probably isn't that much. Some stones that we use for hardscape can add to GH.
Your KH is around 2-3 dKH which is fine, IMO. Is there a reason you want to raise it? You can use sodium bicarb and there are other products sold just for that purpose such as Aquavitro Carbonate.
You mentioned that you're tank is improving which would make me want to continue as I'm doing and not change things up too much. There are plenty of good scapes with hard water, George Farmer's tanks that keeps at his home and for Aquarium Gardens are examples, since he mentions having very hard water at both.
Planted tanks do well with consistency as long as that consistency is in their favor.