I have a Quickie Ti Titanium (over 20 years!) and still going by doing proper maintenance and doing my own repairs, one being dealing with these screws shearing as well as stripping out.

If the screw is sheared you can use a screw extractor kit (can't vouch for this particular one, but there are a bunch of options like this - mine looks identical) to first drill a small recess in the top of the broken screw and using the other side to back it out....

HOWEVER...these will often destroy the threads of the rivnut (the threaded insert in the frame itself) so be prepared to replace the rivnut.

To do so you must determine the screw size and rivnut you will need. I have a set of thread gauges (like this: https://www.amazon.com/CKE-Bolt-Thread-Checker-Metric/dp/B08L38BJDS) and you can remove one of the good screws and verify the size if you don't know. Then get yourself a rivnut kit and GOOD set of drill bits.

If you need to remove the rivnut, start with a bit that fits inside the threads, Move up in size gradually, drilling out the walls of the rivnut until they just shear away, allowing you to remove the damaged rivnut with a pair of needle nose pliers, or similar. You do not want the drill bit to bite into the titanium.

Then you can insert a new rivnut, expand it inside the tube with the rivnut tool and you are as good as new. Here is an example of a rivnut tool You will want to find one that works for your size insert.

Get new stainless steel screws at a quality hardware store or a place like McMaster-Carr (mcmaster.com). Do not buy cheap Chinese junk screws from Amazon. They will just break off again.

It may sound like some specialized tools and equipment but if you depend on your chair and are able to do the work (it's not difficult) it's a worthwhile investment to protect your mobility and independence. And it WILL happen again and you will be able to fix it in half the time next time.