I agree, mentioning "everyone" in the hope "some experts will notice and help" feels like dramatic overkill that at minimum will get someone yelled
at out. (IMHO pinging "everyone" only makes sense in tiny groups/slacks, and
maybe for the rare announcement that is vital everyone see.)
If you want people to have the ability to at-mention a "group of people who cannot help" why not, eg, create a "helpers" or "experts" group who
opt in to being pinged for assistance, and encourage people to use that group instead of everyone/all/here/etc.
https://slack.com/intl/en-gb/help/articles/212906697-Create-a-user-group