I feel useless as a Friar

Unpopular opinion it seems due to the popularity of rejuv friars nowadays, but if you feel useless it's because you went rejuv, end of story. Especially since you already play a rejuv cleric and can (hopefully) see how much better they are at the job.

Go enhance/staff or hybrid; just enough rejuv for first cure NS. It's slower but still better than forcing your cleric(s) to try to cure rather than heal.

What you do depends on your group and the engagement. Sometimes I'll fly in with the dps tank(s) with the slowest staff I have, let them slam and then pop ST and glue myself to an enemy caster or healer. I have to be wary of overextending of course because I'm a glass cannon, so I try to pan around and see what's happening in the fight elsewhere. Sometimes I take my fastest staff and sprint around snaring everything with a weapon drawn. Sometimes I stand in the back and act like I'm a cleric or rejuv friar until I draw melee attention. If it's a train, I ST and bail, if it's 1-2 and I trust my heals, I light them up until they figure out they're wasting their time on the wrong target. When they turn to go for the cleric, back snare and move to either peel more or heal. Especially in tenser fights, you're also using your taunt and ranged dd charge (you hopefully have one) for interrupts. Yes, they're on longer timers, but they at least need to be mentioned since they're instants, especially the taunt. Everything mentioned above requires decent weaponskill though.

To be effective at off-heals with low-ish rejuv requires watching for things before they happen and knowing when someone has positioned themselves poorly. Against hibs for example, I've already started casting my greater heal the moment I see the stun spell effect because if you wait until damage is taken it's too late. Requires a lot of camera panning and looking at the field instead of health bars, but it works.

And this is basically what I think the class was meant for. You're meant to be flexible, even within the span of one fight. This requires you to be fast at adapting, because you're not going to be the absolute best at most of what you do, but you'll be good enough to ensure survival. And I think this is the problem with rejuv friars. They can't do much of anything beyond heal, and they're never going to be as good as clerics at healing despite sacrificing effectiveness at other roles. In theory, a rejuv friar is kinda a 'meh, ok sure' addition to a group. In practice, from what I've seen at least, is that they're even worse because they shirk off all other duties to heal almost exclusively and again, I'd rather just have a cleric do that.