Hows the condition of the plywood?
Ideally you want to run the wood across the joints.

But if you choose not to consider adding extra support to the joists.

House built circa before 1885.
Original flooring was 3/4in 1x6 T&G pine laid across joists.
3/4 in oak T&G (2 1/4 in) added circa 1896, at right angles to the pine.

For unknown reasons, both layers of the living and dining room floors were removed in 1992 and replaced with the 3/4 plywood, and then carpeted over.

The plywood is in good condition.

Adding cross support to the joists would mean removing the plywood to get under it, adding some cross-joists, and re-installing plywood. Judging from the nailing pattern, the people who did the removal did not add any. It's definitely doable, it's just a royal PITA.

I need to check and see if the joists are only where the nailing is or if there are ones between, where they didn't nail. Judging from the way the carpet was installed, they did the minimum amount of work and relied a great deal on gravity.

So right now you just have 3/4 ply over joists?
NWFA i believes says you should add another layer of ply inorder to run parallel with joists.

Most of the time when a customer wants that i add another layer or have them sign a release form acknowledging i warned them.

Thanks. I don't want to risk boards popping if the subfloor flexes between joists.

The decision ... run the new flooring across the joists. Because I don't want to deal with more plywood layers and matching flooring heights.

The SO pointed out that we are going to have wide plank thresholds filling the doorways (12-18" walls!) between the LR/DR and any other flooring so matching directions is not an issue. And seeing 35 feet of boards running the length of the area could be impressive.