With the 1.15 update to TNT that makes it drop all of the blocks it breaks, it has become extremely useful for a variety of farms and redstone-based applications, from concrete-makers to automated wood farms. It's also actually viable as a way to mine now. In order to allow players to take advantage of this, I suggest making it usable in some fashion. Currently, when TNT is placed above-ground, it tells you that TNT will not destroy blocks above sea level.
Perhaps, then, one way to make it useful without making griefing much of an issue would be to make it not work unless it's below sea level. This has a flaw, though, since some people build things underground.
Another way to make TNT usable that would further negate griefing would be to make it usable only in the End and/or Nether, which most people don't build in. Some things are still often built in those dimensions, though, like enderman or pigman farms, and some people might just like building there.
Yet another possible solution is to take advantage of the server's land claim system and make TNT only usable within claimed areas that have /claimexplosions toggled to on. Currently, toggling it does nothing; TNT is still prevented from breaking blocks. This doesn't allow for the use of automated tunnel boring machines, however, since it'd quickly leave the claimed area, but since it would let us use just about every other thing we'd use TNT for, it's probably the best possible solution and it's the one I suggest if it can be made to work while still having TNT disabled in the world outside of these claims.
I don't know if it's possible, but one way could be to restrict TNT usage to a rank that's given to members who are more trusted for whatever reason, perhaps playtime and/or a history of building stuff on the server. That would allow for advanced stuff like tunnel borers or perimeter-creating flying machines while having a minimal griefing risk in my opinion.