Adding Joists to Reduce Deflection for Tile Floor

I need to reduce the deflection for ceramic tile floors in 2 bathrooms located side by side over the same joist span. I have to add the support from underneath because I already finished tiling one of the bathrooms without doing anything to reduce the deflection. The drywall ceiling below needs to be replaced anyway b/c of planned renovations to the kitchen below. I already have some access to view into the joist bay, and only see one wire running through the bay and that wire needs to be moved anyway during the kitchen renovation.

House was built in MD in the 1980's. Joist span is 14' 3", with SPF 2 x 10 joists (9.25" high) spaced 16" on center, topped with 19/32" thick tongue and groove plywood that was nailed and glued to the joists. The two 5' wide bathrooms cover 10' of this joist span.

How should I reduce the deflection?

Could it be done with just blocking or steel bracing or do I need to add additional 14' 3" joists?

If I need joists, should I sister them to the other joists or try to center them to create 8" on center joists? One of them can't be centered b/c of toilet pipes.

When I viewed the joist bay, I expected to see an open end where the end of the joists rest on the interior load bearing wall in the center of my house, but the end was closed. Is that probably blocking, a rim joist, or something else?

Thank you in advance for your help.