Congratulations for the shiny Magicarpe and your Tyranitar! They look great! :D And I’m sorry to hear that you’ve encountered such a traumatising event. I hope you’re getting the help and the rest that you need and you’re feeling better.
For the shield breaker, I agree with other commenters that Tyranitar with Bite + Crunch is a very good generalist against regular Team Rocket grunts (maybe except for the Lapras one), but in TR leader battles, it doesn’t have the best performance because of the shields and the slow energy charge rate of Bite.
Against the current TR leader line-up, you may want to consider these following pokemon to burn their shields:
Scizor with Furry Cutter + X-Scissor/Night Slash is by far the best shield breaker against Sierra’s current team. Furry Cutter + X-Scissor is an excellent combo for shield breaking and dealing SE or neutral damage. Night Slash is also useable if you’re short on TM and candies/dust, as it uses the same amount of energy as X-Scissor to activate, and an occasional buff of Attack is also a nice bonus.
-Lucario with Counter + Power-up Punch + Close Combat/Aura Sphere/Shadow Ball. Its baby discount for 2nd move before evolving is worth considering. Counter + PuP is Lucario’s essential shield breaking moveset. Its second move depends on your TM stock and Lucario’s teammates. Close Combat is the most appealing option as it’s fast, powerful and hits Lapras and Cacturne for SE damage. Its debuff on Defense means it’s a nuke option before Lucario going down. Aura Sphere also hits them for SE damage, but is slower than CC. Shadow Ball hits Kadabra with SE damage, but Lucario may not survive long enough to use it.
-Dragonite with Dragon Breath + Dragon Claw. You can unlock the second move on a Dragonite with exclusive CD move Draco Meteor, so that you can break shield and nuke the next pokemon. Or you can add Outrage on a Dragonite with DB + DC. Both fonction similarly, except that Draco Meteor is more powerful but slightly slower to charge and has a debuff on Attack.
-Flygon with Mud Shot + Dragon Claw is another speedy shield breaker. Compared to Dragonite, it lacks the raw power, but it has better tools (Earthquake and exclusive CD move Earth Power) to deal with Steelix.
-Swampert with Mud Shot + Hydro Cannon/Muddy Water. To break shield, Muddy Water is very fast, but to do meaningful damage, you’ll need to have the exclusive CD move Hydro Cannon, especially since Bagon will resist Water type attacks.
-Poliwrath with Mud Shot + Power-up Punch or Ice Punch. It resists both of Bagon’s fast moves, which allows a more consistent performance. Depending on your TM stock, you can settle on either PuP or Ice Punch for shield breaking. PuP is faster but lacks power, while Ice Punch is slightly slower but allows SE damage to Bagon once shields are down. If you can unlock the second move, then you can consider use PuP and Ice Punch or Dynamic Punch, depending on your TM luck and Poliwarth’s teammates.
-Omastar with Mud Shot + Ancient Power is also pretty fast to break shield, and is especially efficient against Ember Bagon + Charizard + Fire Fang Salamence, but can lack a bit of power against other line-up.
Cliff is probably the most difficult one in the current rotation, because of his Stantler’s hard-hitting fast moves and his other pokemon generally possessing more bulk.
-Lucario with Counter + PuP can work pretty well, but I find that it’ll need to be relatively high level to have a more consistent performance.
-Against Zen Headbutt Stantler, you can get away with Scizor with Furry Cutter + X-Scissor/Night Slash, Metagross with Bullet Punch + Meteor Mash (CD move) or Tyranitar with Bite + Crunch. Scizor is more reliable for shield breaking, while the other two have more power but less consistent to burn shields.
-Against Tackle Stanter, you best bets are double-resisting pokemon such as Giratina altered (with legs) with Dragon Breath + Dragon Claw or Ancient Power, or Probopass with Spark + Rock Slide, or Aggron with Smack Down or Dragon Tail + Heavy Slam. Giratina-A is the most reliable shield breaker in this scenario, while Probopass can break shields but lacks power to take Stantler down, and Aggron has more power but slower to burn shields.
Other pokemon such as Vaporeon (Water Gun + Aqua Tail)), Mamoswine (Powder Snow + Avalanche/Ancient Power (CD move)), Magnezone (Spark + Mirror Shot/Wild Charge), or Blaziken (Counter + Blaze Kick/Blast Burn), Electivire (Thundershock + Thunder Punch/Wild Charge), Melmetal (Thundershock + Rock Slide), etc are all amazing shield breakers, although I find them slightly less optimised for the current TR leader rotation. For example Magnezone works wonder when Arlo had Scizor, but for the current Bagon, its Steel typing is a drawback if it has Ember as its fast move.