The fix is to create a larger gap between the S21 and the charger which can easily be done this way (see picture). The phone will still charge even with a thick case as mine (supcase unicorn beatle). It also prevents scratches on your case or phone if you do not use a case.
Than you are lucky and the only person who has no heating issues due to charging !!!
Just because a phone gets warm during wireless charging doesn't mean there is a problem that needs to be fixed. @smiley has already stated the reasons for this and so I won't waste my time explaining it myself.