What am i doing wrong? first grow very inexperienced.

High, The tips burned and turn yellow is due overfeeding. You did everything really well especially for a first grow!!! When I look at your diarie and your pictures I would have put the light even 10-20 cm lowers! Light is the main dish for plant so the only way to make them fatter is due more light! Happy Growing! 😋

Runtz varieties are known for having small buds. Your plant is doing fine, just keep checking the trichomes(as mentioned before, around 80% cloudy, 20% amber) until it is finished. Happy harvesting!

I can't add anything substantial to what's already been said here... you're nearing the end of their life cycle which is when they DO start to rather "tread water" in terms of growth/bulking and is also when they start changing colors. They look magnificent! Yes, P and K are important at this stage but you're obviously giving that to them... I'd sit back and contemplate how much time you're going to have to devote to trimming! Grab yourself a microscope or jeweler's loupe so you can get in really close to the trichs to see what THEY are doing... I'm betting a lot of them have already turned milky... what you want to see is all of them milky and 10-20% of them amber... that's when you should harvest! Good job!

As tubington9 said. Also your plant is wanting P and K at this late stage.

The bulking could be an issue of light intensity (or lack there of), as per your yelling tips, you are 12 weeks in to an autoflower, the plant is reaching the end of its life cycle. Not sure what nutrients and what not you are using but at this stage the plants are generally very hungry to assist with bulking. All in all it looks good for a first grow.