I don't see that in it's
constitution. From Wikipedia quoting their constitution "The WHO Constitution, which establishes the agency's governing structure and principles, states its main objective as "the attainment by all peoples of the highest possible level of health." The WHO have had huge success and I think you are misunderstanding its role and how it operates. It isn't set as a world health police but is just a specialist unit of the UN but it is mainly a managerial bureaucracy and will need some redesign if it is to become the pandemic response unit. It has had many great successes and the world would be far poorer (and fewer in number) without it. If you said it is too bureaucratic, sensitive to politics and slow moving in general I'd agree, but to lay the pandemic at its feet is drawing a very long bow. There was more than enough information out there for governments (who should be making the actual decisions) to act in time, as did a few countries, but to lay the blame for the incompetence of many government at the feet of the WHO isn't fair or reasonable.
Taiwan not being part of the WHO is because it isn't part of the UN which is tied up with relations with China. The WHO can't decide who is part of the UN and in any case what that has to do with the WHO's pandemic response isn't clear. And I do blame trump for cutting funding to the WHO because it was a purely political act. If you want them to change keep funding and negotiate KPI's. Don't cut them off at the knees mid pandemic. Like everything Trump did that was idiotic and hopefully Biden will reveres it ASAP.