I don't see what would be wrong, if anything, in asking God for and receive physical healing. If by preaching "prosperity" means getting filthy rich or even rich from gospel preaching, then, yes, that would be wrong. But if "prosperity" just means having your basic needs met for health and sustaining life, then np.
The main issue I see is that, when one believes in say Psalms 91, and if they happen to get covid-19 for example, well meaning Christians might mislead them to think its because their faith is not enough, they did not tithe etc etc, which I find very harmful.
I would instead stick to the Apostle Paul and teach them to look forward to the rapture, in the midst of this, as stated in Romans 8:18
18 For I reckon that
the sufferings of this present time are not worthy to be compared with the glory which shall be revealed in us.
19 For the earnest expectation of the creature waiteth for the manifestation of the sons of God.
20 For the creature was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope,
21 Because the creature itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God.
22 For we know that the whole creation groaneth and travaileth in pain together until now.
23 And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body.
24 For we are saved by hope: but hope that is seen is not hope: for what a man seeth, why doth he yet hope for?
25 But if we hope for that we see not, then do we with patience wait for it.