I have gathering profession (herb+mining) on my main character (farming during waiting in que times) and I am not sure if I should just sell materials or make alts for alch+ insc and bs+ jwc.
Also if I would make alts for professions, do they need to be lvl 80 to be worth?
Your alts don’t need to be level 80. I don’t know what is the minimum level requirement but level 70 is fine.
As for selling vs crafting, selling the materials is the easy path and you will earn a decent amount of gold.
Leveling crafting professions requires spending a lot of materials (or gold) in order to be able to craft things at a high quality, and you will be competing with other crafters who are way ahead of you. There is a mechanic allowing you to catch up with them, but it will take some time and a lot of gold. Crating doesn’t pay off immediately and requires a lot of investment.