I'm really going to try to sell you on Mega Tyranitar here. It's the best teammate for Excadrill. It sets Stealth Rocks for it and helps deal with some of the bulky flying types stopping it from sweeping. For example, it can lure in Landorus-T and kill it with an ice move. It can weaken Celesteela which is needed to facilitate the Drill sweep and is something you mentioned you had a hard time against. I don't quite understand your concern for not wanting to use Mega Tyranitar unless you really wanted to use Mega Garchomp over it instead. Base Tyranitar just isn't that optimal since its Mega provides enough offensive pressure to not need a choice band and provides additional utility with hazard setting. If anything, go with an Assault Vest set instead as it helps check Mega Charizard Y and Mega Alakazam for Excadrill. Eventually we'll end up with two different versions of the team with different megas because I really think Mega Tyranitar is worth trying.
Tyranitar @ Tyranitarite
Ability: Sand Stream
EVs: 132 HP / 252 Atk / 124 Spe
Adamant Nature
- Stealth Rock
- Stone Edge
- Pursuit
- Fire Punch / Ice Punch
The Speed EVs are to outspeed uninvested Rotom-Wash. If you aren't interested in that, you can go with 76 EVs to outspeed uninvested base 80s such as Mega Venusaur and put anything leftover in HP. The final moveslot could go either way whether you want to be able to hit steel types that are 4x weak like Ferrothorn or if you want to lure Landorus with it.
Tapu Lele @ Choice Specs
Ability: Psychic Surge
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Psychic
- Psyshock
- Moonblast
- Hidden Power [Fire]
I like Tapu Lele here as it can safeguard Excadrill from priority moves that it really doesn't want to take such as Water Shuriken from Greninja and Sucker Punch from Mega Mawile. I would go with a Choice Specs over the other items as you have Sand Rush for speed already and I think Excadrill would rather have the Z move as I will soon explain. Personally, I would take off Thunderbolt since specially defensive Celesteela takes it surprisingly well and will be able to take advantage of the fact that you are locked into an electric move by getting a free switch-in to a ground type. You could go with Focus Blast instead to nuke Heatran, but I think having Psychic to hit as hard as possible would be best. Plus, it 2HKOs offensive Heatran anyways and you don't have to risk missing.
Excadrill @ Steelium Z
Ability: Sand Rush
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Swords Dance
- Earthquake
- Iron Head
- Rapid Spin
Swords Dance is a must have on Excadrill. In combination with Sand Rush, it is extremely dangerous. I'm not sure what Poison Jab is for. It's not doing much damage to Tangrowth even with an attack boost. Though, with the help of Steelium Z, it can easily break through it as well as some of its other usual checks after a Swords Dance. Excadrill is a great Z user. You were talking about needing a Pokemon for hazard control, but you have Excadrill right here with Rapid Spin. It would come at the cost of getting rid of Rock Slide, but it doesn't do much at all for you compared to the utility that Rapid Spin provides so it's a small price to pay.
I have detailed about how Mega Garchomp competes with Mega Tyranitar for a slot already, but it fights for Excadrill's spot as well. They occupy similar roles and I think you are better off with something else if you do decide to go with Mega Chomp. I just don't think they can both fit on the same team. Since you would lose out on Sand Rush, you can change Tapu Lele's item to be scarfed to help with speed. Tornadus-T would be a great alternative as it is another great Z user with hazard control and you were already considering adding it.
Sharkeisha (Garchomp-Mega) @ Garchompite
Ability: Sand Force
EVs: 4 Atk / 252 SpA / 252 Spe
Mild Nature
- Draco Meteor
- Earthquake
- Fire Blast
- Stealth Rock
The set you have already is great, though I would suggest putting Stealth Rocks on it. It forces a lot of switches so you will have ample opportunities to set them up, similarly to how Excadrill finds opportunities to spin. This way you wouldn't have to put them on Tyranitar meaning you are free to use Assault Vest on it. I don't think Stone Edge is necessary as you have Fire Blast to hit Celesteela and Draco Meteor will be doing plenty to flying types, so you can replace that move.
At this point, the team could be looking one of two ways right now depending on whether you want to go with Mega Garchomp or not. I can completely understand since the team was originally built around it after all. Toxapex is a great fit regardless of which route you decide to go with the team. Ferrothorn forms a strong utility core with Mega Garchomp, providing Spikes to complement its Stealth Rock setting. Garchomp also enjoys the Thunder Wave and Knock Off support that Ferrothorn can provide. By now, there are loads of other possibilities on what you could go with. The reason why I linked so many pokepaste sets is because there is a lot of mixing and matching you can do with these sets. I know I didn't really answer your question so much as improve what you already had, so here is a final sample team. Feel free to switch anything up, like for example you can always go and put Excadrill back over Tornadus-T if you don't like that change. Don't be afraid to try out Mega Tyranitar, by the way.
