I have that small portable toilet in my van. I recommend also a sturdy box to go underneath it. It's very low by itself, for an 89 year old. Or for me!
It has a water reservoir and a flushing mechanism, but like most of the commenters on Amazon, I don't use that. You can if you want, though. I find it more convenient just to pour a little water out of a jug if needed.
I previously used the bucket, pool noodle (or pipe insulation), and garbage bag system. Honestly, for intermittent use I would find that more comfortable and convenient. It would depend on her bowels' level of persnicketiness. And also her balance. The bucket requires a little more balance than the potty.
For poop, you don't need the kitty litter. For pee, I used a plastic jug which I set inside the bucket, with also a clean garbage bag in the bucket. As backup for for poor aim :-). No kitty litter regardless.
Add the portable shower tent for an enclosure, and things should be pretty ok.