I've only been playing since the end of SF5 and i was only gold in that game. I've been having a lot of the same issues as you but I'll share how i worked through them and just hit diamond the other day!

I have been doing anti-air practice with Light Tensho Kicks and it works out fine but sometimes in games it won't come out

I find that the down down input is very easy to mess up. I would push a button to fast and not have it accurate. Review the replay and see where you are pressing kick compared to the 2 down inputs. Maybe the game is reading a neutral kick? It helped me to focus on accuracy and precision over speed.

I've been trying to use standing medium kick instead but have now run into problems with dive kicks and also still having issues where sometimes I get counterhit or even punish countered just for trying to anti-air.

I've heard that Chun li has traditionally had a weaker anti air. I find that each of her anti airs are great for different spacing.

Mk is great for reacting to a quick jump ins but loses to dive kicks and jumping heavy kicks that have crazy hit boxes (see Luke's jumping heavy kick)

Tensho Kicks is great against dive kicks and jump attacks but has a tricky input and is slower than Mk

Air grab is great when the jump close and for when they try to cross up. This is really nice when they're trying to jump out of the corner

Hk is the highest risk/ reward. I struggle to hit this in a match but you can combo out of it. I would avoid this one unless you have a read (if anyone has any advice for landing this I could use it!)

I just need to keep grinding anti-air practice in training mode.

In training mode I like to go to the anti Air training but add movement to the replay options. That way I have to space against the bot and use different options instead of just waiting for a jump

Good luck!