Any suggestions, how to improve my PvP Great League Venusaur centered team?

Mine is:
1. Glalie IS/Av
2. Ivysaur VW/SB
3. Solrock RT/RS
I don't have time to buy second charge moves for them yet. I sometimes switch out Ivysaur for MS/Eq Flygon to counter Registeel. As for your team, I think it's great. It's able to counter the legendary Birds and Beasts, and I think it covers every single type.

I don't PvP much, but wouldn't a Sceptile work well, too? Get the 2nd move for Earthquake for the cheap 10k dust. Fury cutter a desirable fast move. Some people would be surprised about it bc most are saving candy for a community day. I haven't done it yet but I have three at 2500cp and I still have 1800 candy to spare, plus 6 Treecko luckies.

Maybe half the time use your Venusaur and half the time use a grass Meganium w/ EQ or a grass Sceptile w/ EQ to try to neutralize any common fire counter.

It's a good team, the only weakness I can spot is Ice as 2 of your Pokemon are weak to it. You do have SE on Flygon as coverage but from my experience it usually faint before it can charge one against ice fast move. You can try changing lanturn to Charizard with FS/BB+DC and see if it works better.

Why are you using Stone Edge on a Ground/Dragon pokemon like Flygon? Dragon Claw is great, but in PvP Earthquake is amazing. It's still slow, but then, so are the other one bar moves, it does a lot of damage, and gets powered up fast enough by Flygon's Mud Shot, which generates energy crazy fast (as you well know).

Further, Earthquake hits a lot of foes for SE damage, like Metagross, Tyranitar, Steelix, Entei, Raikou. The Ground typing compliments the Dragon typing nicely, with Dragon covering Grass and Flying nicely (giving resistance to and avoiding resistance from).

Stone edge is good too, covering Flying well, but doesn't provide STAB, so it's weaker that way, and Flygon isn't an attacking powerhouse, so getting every edge in damage helps.

Did you mean Solar Beam or sludge Bomb? If solar beam, TM to Sludge Bomb for coverage.

I run a team as such for Great:
Lucario: BP+SB
Venusaur: VW+FP
Kingdra: WG+Bl

Kingdra could do with buying Outrage, but otherwise a very solid team. From what I've learned, you need to cover any two pokemon that share a weakness: Ice (say, from my Kingdra) would rip apart two of your mon, so I would worry about a Piloswine or similar. You don't necessarily need to counter your counters, but at least be able to ensure a neutral damage situation.

You're very strong against Azumarill and other water types, but a bit soft against Altaria and Cresselia. Your best swap for both is Flygon, which isn't the greatest counter. I like Lanturn because it means you're not weak against Skarmory, which would counter both Venusaur and Flygon very efficiently.

But it depends on what you're actually seeing other people bring in. Take note of when you don't have a great swap-in and modify your team based on that.