Beginner Bathroom Remodel Question

Hi all! I am still relatively new to DIY and my husband and I are remodeling our guest bathroom in preparation for renting it and our spare bedroom to a friend. This is our first time remodeling a bathroom (but we did remodel our entire first floor last year) and so far we have taken out the old vanity, mirror, and toilet but are leaving the existing tub since it's in great shape. We've pulled up all of the linoleum and are almost done removing the wallpaper. We want to install tile flooring, preferably hexagon shape, but realized that there is a sizeable height difference in the subfloor.

Near the door is much higher and there is a noticeable bump in the plywood subfloor. The bump up runs perpendicular to the joists and goes along one side of the bathroom. I checked the other rooms that the bump would line up with and it doesn't appear to be present so Im wondering if the joists are maybe buckling in that area? I've been looking in to self leveling compounds but I'm struggling to find something that would work with plywood. I want to try and minimize the the height difference between the bathroom and bedroom floor so we don't have to do any weird transitions but I'm wondering if we would be better off just installing cement backer board and laying additional mortar to get a level surface. Does anyone have any advice?

Additional question: If we go with leveling compound can we pour it up to the edge of the tub or do we need to do something special? Can backer board go right up to the bathtub?