300 meters isn't all THAT high, so I think your goal is achievable. Do you need to carry a payload? I'm guessing the following rocket can get that high, if you do everything right:
https://www.jamesyawn.net/modelrocket/intro/index.html
Note that the materials are nothing exotic.
I think light wood is often a superior material to PLA or cardboard. Older kits from Estes used to use balsa for the nose cones and fins. If you don't have balsa, do you have Paulownia? It's not as light as balsa, but it's lighter than just about everything else. Even if you don't have these, I'm sure ordinary light woods can be used. You could, for instance, hollow out wood nose cones. Thin wood over Styrofoam is also a good combination.
You can make your own motors and propellants. But I suggest you don't try unless you are a careful person and are willing to observe appropriate precautions.
I'm not real keen on cooking mixed propellant, but I think using dextrose monohydrate or certain other sugars minimizes the temperature compared to sucrose. If I'm not mistaken, ordinary corn syrup, as opposed to the high-fructose type, is mostly dextrose. Of course, I don't know if you have corn syrup in Pakistan. But there's probably something. Information on a number of different sugars that you can use in fuel is found here:
https://www.nakka-rocketry.net/dex.html
Mr. Nakka has developed propellants using potassium nitrate and epoxy, which don't have to be heated, though they may be a bit harder to ignite than some. I think these are safer. If you haven't already, you will find much useful information on his site.
In the US, Spectracide "stump remover" is usable potassium nitrate. (They should probably call it stump rotter.) Potassium is also used as fertilizer. And Mr. Nakka's site describes a couple of ways to make it from other substances.
If you make your own propellant, please be careful. We want you to have the same number of fingers and eyes next year that you have now. And the year after that, indefinitely.
I'd guess that smaller sized motors with cardboard casings may be less prone to bursting, and certainly they will be less dangerous than larger motors from other materials.
I don't have any idea how likely your proposed activities are likely to arouse suspicion where you are, but it's something to think about. You can get to 300 meters, and more, with water rockets, but that's a different hobby. I think the record for those is around 1,000 meters at the moment.
