Are there any considerations to take into account when trying to design your own nozzle? I learnt many things from Nakka Rocketry, however, didn't find many information regarding rules of thumbs for nozzles.
I assume you’ve seen all the usual articles, like this Wikipedia article: https://en.m.wikipedia.org/wiki/De_Laval_nozzle.
Your next step would probably be to look at the bibliography for the Wikipedia article to find the source material.
Practical nozzles that are conical (not the optimal "bell" shape) are only about 2-3% less efficient, in general. A 15 degree expansion cone (1/2 angle) is typical, with 30-45 degree entrance cone (1/2 angle). The throat cylindrical length should be <0.4 times the throat's diameter.
If you choose ablative material, it is more difficult to design the grain geometry for the desired thrust curve. Hardwood, phenolics or high-temp thermoplastic open up as the motor burns, reducing the Kn as it goes. Graphite of a high-grade, high-density, will be essentially constant over many firings. Machine of graphite is messy, though. Ceramics are tricky because of the thermal shock, and the difficulty in molding them and allowing for shrinkage during kiln drying. Steel nozzles are difficult to machine, and can become a dangerous projectile in a motor failure.
For larger, longer-burn motors, the thermal load on the nozzle may cause it to fail. It will also transfer heat into the casing and may cause the casing to fail. A good design choice is to have a phenolic insulator to hold a graphite insert. Also avoid sharp edges in the graphite shape because it will introduce stress points that may shear off under heat and pressure.