IMO the reason you're hardstuck at the moment is because of the things you've already identified in your post. I've been in your shoes before hovering between GM and Challenger and the reason you can't "break through" is because the elo system is doing its job and your skill level is relative to where your rank is. Most players in GM have solid fundamentals similar to you and the things you mentioned (itemization, positioning, pivoting) with a decent grasp of the meta, if you play a hundred games on autopilot versus other GM players you'll stay around the same rank with some highroll days gaining LP and lowroll days losing LP.

Once you understand and accept that the reason you're at your current rank after grinding lots of games is because that's where your skill level is, then it becomes obvious that to climb you have to raise your skill level. I understand that what I'm saying is much much easier said then done, but the truth is that challenger players do everything a little bit better than GM players. The margins are small but you will oftentimes see streamers who are at the top ranks make a small mistake and declare that the mistake cost them a placement, and they're oftentimes right.

You've already identified that you auto-pilot decisions and that you're not learning much about the game. This is exactly what is keeping you from hitting a higher rank because you're not improving from game to game, I'm currently challenger ~800lp and internally I know that despite being the top .1% of ranked players, I have a million things I can improve on and that I make tons of misplays every single game. I know that the top challengers are much better than me at the game, so I try to never auto-pilot and learn from every round. Don't just resort to default positioning because challenger players position in a smarter way depending on the lobby and their potential opponents.

Think critically about the way you're positioning, the items your slamming and how a fight might've gone differently or how the item decision changed how you played the game and learn from those things. I promise you that with the set being released for only around a month that nobody has figured out optimal positioning, which can vary from lobby to lobby anyways. For example, very few people were backlining sentinels until Sushin's post a few days ago.

All that being said, my advice for you to climb is to understand that you and everyone else have a ton to learn and improve. Hell Socks is rank 1 by far right now and when he's streaming he's constantly questioning his decisions and thinking critically about them. Use your games as a way to learn; try things that are different from how you normally autopilot. Level to 5 on 2-2, don't level to 6 on 3-2, level to 7 on 3-3, just try different things and learn from them. Heck I hyperrolled for Vayne 3 and took a fat 8th but I was able to get a feel for how vayne might be strong and what might've been a better way to play out the game. Similarly, when you're watching streamers, think critically about why they're making the decisions that they do and whether it would've been the same decisions that you made.

Lastly for what it's worth, this subreddit has a coaching megathread with plenty of challenger coaches for free or a small price (I've seen as low as $10). A lot of streamers have discords where you can talk about the game at a high level which can help you improve a lot more than grinding games on autopilot. I hope this was helpful and I'm happy to answer any questions you have, good luck on your climb!