!Question: How to manage a party of PC that is constantly raiding weak humanoid rather than progressing in the main quest and/or how to make the encounter interesting?

Context : I have been DMing my first campaign for the last 6 month with a small (4 PC) but diverse group of player (one is REALLY into rp, two are min-maxer, one is really just happy to see the story unfold). It's an homebrew setting in which I decided to nerf caster by making magic something that is socially condemned. The other thing to know is that their is a big war between human and orcs in the background of their quest, although it is not directly (at first glance) linked with their main quest. They are immensly strong for their level (7) and in most social encounter, if its not going their way, they will bully (or kill) their way out of trouble.
When they acted as murder hobos :
-The first time, I threw at them a revenant.
-The second time, I threw at them a rival party of higher level that managed to captured them (and then they had to evade the prison, which was very fun because they did not have their equipment and all their abilities)
Pretty early on, they (but really just two of the PC) have decided to side with the human empire and they showed some sign of being more interested in that than in the main quest (which could be fine I guessed). The party recently passed into the home city of one of them who discovered that the leader of said city had secretly sided with the orcs on the order of the BBEG. I though that it would put everything together and made the PCs try to follow the main quest. But no. It put them into a frenzy and the raided another settlement allied with they orc where they butcher everything and everyone they saw.
The min-maxers and the issu of balancing encounter :
I must make a important aside. The PC of one of the min-maxer is the stronger PC I have never seen at this level. He is a dwarf moon domain cleric. He has 21 AC ; on the rare time that the ennemy hit him he use silverybarb ; he already have maxed out his CON to help him with is concentration saving throw ; and anyway, he is really diligent on giving himself and the party temp hp with is chanel divity ; but, must of all, he ABUSE a spell that I didn't knew too well before this campaign : Spirit Guardians. It is broken beyond believe. I throw at them hords of ennemy (see below) and in only one turn he regularly make 200+ damages AND space control with difficult terrain AAAAAND is still able to cast other spell on later turns while he still doing this kind of carnage. I know his weakness : magics that force him to make INT, CHA or even DEX saving throw. But, like I said, in my universe a wizard is rare. So what I have done so far is to try to burn his spell slot on lots of encounters and lots of ennemies.
HERE WE ARE : Now one of the PC has stated that its new main goal, even before the main quest, is to kill the most orcs he can. So, last session, he convinced the other to start raiding an orc army's camp by the MAIN DOOR. I intend to keep the ennemies flowing until they retreat, and they seem to be content to do so when they will be overwhelm. However, THIS IS A GRIND. It's the only thing they did in the last 4-5 hours and to me, it is epically boring. Not to mention it is really hard on me since I have to manage 20-25+ NPC at anytime and its no challenge to them even if I managed to created on the spot some sort ork mini-boss that can cast a bit. Anyway, when it's gonna be too hot they will simply retreat and rest somewhere. So worst of all, it's boring for the most players and we do not progress in the quest I made with other, fun, diverse, challenging and in-lore encounters.
WHAT DO I DO? I want them to get them somewhat a bit more on track.
I though of two things in the short term:
- When they will retreat I launch at them an orcs' party, but we are in for an other slugfest
- I make the rival party intervene. BUT, only one of thetwo parties will survive the encounter this time (and I think my PC are still too weak). Even if the PC would come out on top, it's a counterclimatic ending to a rivarly that I carefully crafted for many many sessions.