i have over 180 games on sion overall, and an 80% wr on him this season (ranked) i have watched a lot of sion gameplay (16 month baus sub lol) and i'd like to think im pretty good at the game. most of the other guides on sion are rarely actuallly updated. if you look at them they are really not getting updated the people making them just tag them as updated. i am constantly updating this guide every patch.
im also the only one that actually plays sion a lot.

if you don't watch sion or singed gameplay there is a chance you don't know what proxying is, it is going in between enemy turrets to catch the wave before it meets yours. you can watch guides on how to do this effectively but it is very strong, especially on sion. it also helps you to take enemy camps and increase your cs.

the short answer is, tank, lethality currently just isn't very good. and tank is really strong right now.

cull is an extremely underused item, it essentially gives you 160 free gold, it is by far the most gold effective item in the game seeing as it essentially prints gold. it also is useful for delaying spikes in your build.

corrupting potions are really powerful with time warp tonic especially because to the fact that you can essentially get a free movement speed ability as when you use a potion you get extra movement speed, I like to try to hang on to corrupting potions until the last item

be careful using your ult if there is an enemy champiob that can create terrain like anivia, if they react and use it it will block your ult.

You can use W while using Unstoppable Onslaught to be able to recast for damage once you hit your ult. (or while moving past someone)

enemies cant see your Q if you are out of vision.

You can use your ultimate on towers. (deals a lot of damage if you built ad)

Use your E before Q or R to land them pretty consistently.

Use soul furnace after your enemy uses their full combo so you can still get the recast in. (only really necessary in lane)

In difficult matchups, walk up to the wave at level one-two, which will likely lead to your death, in which case you can get a wave or two and do some damage to the enemy in your passive, making them recall when you teleport back, letting you control the wave. (generally gets you a cs/xp lead especially if they have ignite instead of tp)

sion has been a pretty solid pick for a long time due to his extreme tankiness. so far preseason 13 looks REALLY good for sion with the new tank mythics its likely that he will be as strong if not stronger next season.

https://www.youtube.com/shorts/ZIulAb8WAc8

nasus is my favorite matchup whenever i see one im always giddy with excitement to pick sion, at no point can he contest you levels one through five. i recommend to play this lane very aggressively but there are two ways you can go about playing it, the first way is to freeze the wave under your turret and try to deny him cs while also being able to have your jungler farm him for kills. the second way is to perma shove under the tower and proxy waves while taking the enemy junglers camps, this is what i like to do but both are VERY good.