First, I want to mention that the biggest rival to Groudon teams are, unsurprisingly, Kyogre. It's the second most used restricted legend behind Zacian. Most ogre teams like to lead Tornadus + Kyogre to set up Tailwind and Water Spout, and Tornadus often runs Rain Dance so that it can keep control of weather. Even with Venusaur, I think this team will struggle against TornOgre leads. I'll address this as I go through your team members.
Groudon
Your Groudon set is good, but while it wins a 1v1 with Zacian (assuming Precipice Blades lands), most Zacian players aren't going to let you 1v1 it. Behemoth Blade chunks you for about 70%, meaning that just a little chip damage beforehand and Zacian outspeeds and cleans your Groudon. Ho-Oh is probably better for dealing with Zacian, since Wild Charge is quite rare and it resists all of Zacian's other moves.
As far as the Groudon set itself, I would consider running Swords Dance > Fire Punch and Sitrus / Figy Berry > Life Orb, but the set you're using now is perfectly fine. Just know that you can't play it as an insta-win vs Zacian.
Zoroark
I also have a soft spot for Zoroark in competitive. On the plus side, Zoroark acts as pretty good Calyrex repellant from team preview, since it prevents the opponent from playing safely with it until they can confirm it's not on the field. I also like Taunt on it, since your opponent will be looking out for a disguised glass cannon and may be less prepared for an unexpected Taunt.
Unfortunately, that's about the extent of Zoroark's positives. It absolutely needs a Sash, but it also really wants a damage-boosting item, and while 105 speed is good, it isn't excellent, so instead of the glass cannon it wants to be, it's more like a glass handgun. Options for a replacement would be Moltres-Galar or Urshifu-Dark. Moltres is very bulky and is an excellent set-up sweeper with Nasty Plot, but it overlaps weaknesses with a few of your Pokemon. Urshifu gets Taunt and Sucker Punch, although some Calyrex run Draining Kiss specifically for it, especially on teams with Psychic Terrain where it's protected from SP.
Gastrodon
Very good pick for this team, and the moveset looks good. It would be worth looking at minimizing its speed, since it's already going to go last against non-Trick Room teams, but its slow enough to take advantage of opposing TR if needed.
Porygon2
I don't think your team needs Porygon2. Its main niche is as a TR setter, but you're not using it for Trick Room, and pretty much any dark type is a better counter for Calyrex-S. Two Pokemon that you might want to consider instead are Rillaboom or Regieleki, which are both great at handling Calyrex and Kyogre.
Rillaboom has Fake Out, Knock Off, and, most importantly, priority Grassy Glide when in Grassy Terrain, which it sets. Glide outspeeds and does about 60% to Calyrex, and it's a guaranteed OHKO on speedy variants on Kyogre, while still doing at least 87% on bulky variants (with a chance to OHKO).
Regieleki offers some of the best speed control in the game with Electroweb, allowing your teammates to deal with Calyrex before it can even move.
Venusaur
In my experience with using Venusaur, Coba Berry is absolutely the best damage-reducing option on it, since Max Airstreams are very common and hit very hard. If you feel inclined, you might want to try running 212 Spe EVs instead, with the rest in bulk. 212+timid in sun guarantees you outspeed Regieleki, but does run the risk of getting outsped in a mirror match, so you'll have to decide if the extra bulk is worth that. Other than that, the set looks good.
Ho-Oh
Is there a reason why you have that particular EV spread? If you're looking for a bulkier EV spread, I would recommend investing in physical Def, since its SpD is already so high, although your current spread is already very bulky. There's a lot of options for the EV spread that could work, I'd be inclined to go 60 HP / 252 Atk / 196 Spe / Jolly to outspeed Zacian after one Max Airstream, but something bulkier like 204 HP / 156 Atk / 148 Def / Adamant is also good, or you can stick to what you already have if I'm not seeing what it's been keyed towards. For Ho-Oh, I wouldn't recommend Spe EVs unless you have something in mind you want to outspeed.
Hope his helps.
