Some users have poor video quality but good on sharing

There are many factors at play here (both at the call level and endpoint level), but one important concept to grasp is how shared content and cameras/participant-video are treated very differently in conferencing platforms like Zoom.

Generally speaking, content quality is prioritized, is less time-sensitive, can be cached, and takes up less bandwidth despite a frequently higher resolution. You may see a ~150kbps stream for 4K content and ~6Mbps stream for participant video, as an example. This is because the content (with default share settings) does not need to refresh as frequently and can be compressed more heavily.

It is not unusual at all for content quality to be superb when video quality suffers.

Regarding your specific problem, it makes sense to comb the meeting's dashboard for anomalies surrounding the two participants who had difficulty AND to look into logs on their individual machines. It's possible that high CPU utilization, lack of memory, etc, may have contributed to those users' issues - both of these root causes are completely unrelated to bandwidth, which is only one small piece of the call-quality puzzle.

Also note that Wi-Fi speeds advertised by ISPs are theoretical, don't account for shared bandwidth on a network, etc., and that ALL wireless speed tests are based on signal strength at the exact moment in time the test is run (you may have terrible interference that cripples bandwidth 5 minutes later). ALWAYS use a wired connection for real-time traffic (audio, video...) if possible.