I did! I was mainly involved from 2009-2016ish. I've kept in the loop, played some competitive games online in the games after Sun & Moon, but haven't had the time to take it as seriously these days. I am thinking about giving it a whirl one last time when we get the 10th generation and maybe put in the effort to go to Worlds for the 30th anniversary in 2026.
For a decent stretch of time I was probably one of the best players from the UK, but the game has grown massively now compared to when I played! I got top 8, top 16, top 32 in the UK Nationals and had some good runs through the Regional and smaller events.
What's odd is I think the resources were slightly better at this middling period between when I was starting to tail off and the new age, which shouldn't be the case. Pokemon YouTubers were barely a thing even 10 years ago. Some still put out good content, but I feel like not many do much these days in terms of utility for new players to learn the basics. Somewhat understandable I suppose, as to keep things ticking over they'll be focusing on current things.
However, when new games come out, that seems to be when people go back to making tutorial style videos (as new players are more likely to take up competitive play and old players will want to know how/where is best to train Pokemon). You can probably search through some old videos from around the time Scarlet/Violet came out to get things like EV Training, breeding, catching, Hyper Training advice etc. However, a lot of that is quite straightforward these days and it will be more about team building and decision making. I think a lot of players will try to charge for coaching nowadays, but I can signpost to some players.
For every day content that caters to everything, my friend Lee has one of the best channels these days IMO. His channel is called Osirus Studios. He has made lots of guides in the past, but tends to focus on strategies to beat the 7-star Raid Bosses lately as that's the only real ongoing content in the games.
Aaron 'Cybertron' Zheng does a really good series where he takes teams made by other good players, talks about the strategy, strengths and weaknesses, and then plays a few games with the team so you can see how he'd made decisions. He was a very good player back when I was playing. Probably still is now.
Wolfe Glick makes really good content now and he's a previous World Champion and runner-up. I think he's one of only two players who has made the final on more than one occasion. Loads of other accomplishments etc.
As for websites, this is where it gets a bit more scarce now. I used to work on a site called Nugget Bridge that had lots of good stuff on it and a forum. But internet forums and websites just seemed to keep falling away, a lot of VGC discussion moves to Twitter which kind of sucks. It's hard to explain why, but that seems to be everyone's preference. I think it's just oldies like me that like everything organized in a forum
https://victoryroadvgc.com/
This is the only website I know of that keeps things a bit up to date. I can still probably help with some general advice and how to train particular Pokemon.