It is rather simple to make a fishing alt. You need to fill out the elf questline up to Regicide (for access
OSRS gold to Zul-Andra). Then you want 87 fishing (you are able to do the slow + rewarding procedure of monkfish/karambwan/infernal eels depending because of stats), the ideal way would be barb fishing (which you can do before the quests to find strength and agility levels that is beneficial for underground move / regicide). You then want 88 cooking (for 6 scales per eel) or 96 cooking for 2 drops per eel. Doing wines or anything cooking method that you prefer doesn't take long.
Finally it takes a good quantity to
buy RuneScape Mobile gold of time to make this type of alt (somewhere around 110 hours, 90 for fishing without tick manipulation, 15 for cooking, and another 5 to your quests), but the coaching process is also quite afk. Gain per hour is a hour depending on your level. While I do stuff about the main/iron I have been doing them. Honest question, is there a reason why you do not turn it to a brutal dragon alt? It's in profit with a DHCB like 800k/hr. Obviously the gear would cost more then that you're using, but in case you've got it, then why not? Obviously you'd need 2 more slayer lvls but that would not take.