I'm not too knowledgeable about sun teams, so I can't really give this a proper rate, but right off the bat, I noticed that you're using quite a few pokemon that are outclassed in the OU metagame. Specifically, Umbreon, Machamp, and Cradily. With Umbreon and Cradily, there are much better defensive pokemon in the metagame and with Machamp, it's outclassed by other fighting types like Mega Medicham. I would replace those 3 with other, more viable pokemon. Keep in mind when thinking of replacements that sun teams are generally very offensive teams that pack very little defensive utility.
Also, sun teams desperately need forms of hazard removal since they typically run several Fire type pokemon that are weak to Stealth Rock and the fact that your team doesn't have any would definitely explain why you've been having trouble with that move. I often see Torkoal used as the sun setter for this type of team over Charizard Y, possibly due to its ability to role compress by providing both Stealth Rock and Rapid Spin as well as Toxic to cripple potential switch-ins to your sun abusers. I would hesitate to say that you have to use it, but you can always give it a shot and see how it goes. In any case, I highly recommend adding something that can remove hazards for your team.
And speaking of sun abusers, I'd recommend adding another one besides Venusaur, as like I said, these teams are pretty offensive, so you want a lot of offensive presence that takes advantage of the weather. An offensive Fire type could be a good inclusion.
I think sun + Alolan-Golem is a really cool idea that could potentially work well, but the main source of its potential is in Alolan-Golem's Magnet Pull ability. It gives it a niche over Magnezone by being able to trap specific Steel types like Heatran that Magnezone can't deal with. Heatran can be a big defensive threat to sun teams due to its ability to sponge Fire type attacks with its Flash Fire ability, so having something that can trap and remove it can be really beneficial. In sum, I'd definitely use Magnet Pull over Sturdy for Golem's ability.
And finally, for the remaining pokemon, I'd use the standard Smogon sets over what you have since there's some odd moves and spreads that you're using. For instance, I have no idea what those specific Speed and SpD EVs on Venusaur do. I usually see just 252 SpA / 252 Spe / 4 SpD with a Life Orb for its item to make it a very powerful sun sweeper. Also, I'd make its moveset Giga Drain / Sludge Bomb / Hidden Power Fire / Growth. Growth gives you +2 SpA in the sun which makes Venusaur very deadly. I prefer Giga Drain as the Grass STAB since it's still powerful at +2 and helps you recover off the damage from Life Orb. I guess you could use Leaf Storm for more power, but with Giga Drain, you can keep Venusaur in for several turns rather than firing off a powerful attack and then switching out, so that's why I like Giga Drain over Leaf Storm. I'd say both of those moves are better than Solar Beam for it because they're both usable outside sun, unlike Solar Beam where you don't want to have to deal with the drawback of wasting a turn before you can use it. Hidden Power Fire hits stuff like Ferrothorn that takes little damage from your Grass move and is immune to Poison. Dragon Pulse on Charizard-Y doesn't hit anything important and it would rather have Focus Blast over it to hit Heatran and Tyranitar. In general, it's better to provide sun through pokemon with the Drought ability than to use Sunny Day over a more useful move.
Hopefully this helps you make some improvements to your team!
