Having a lot of alts at a semi high Level can be a very nice complement to your main goldmaking empire. There are plenty of lower level recipes that can bring in a nice profit including daily cooldowns from alchemy and engineering. So let’s look at your options.
Obviously max level professions are best
Once you have all the professions at max the extra value is a lot lower though. There are very few daily cooldowns or similar mechanics in BfA, so having duplicate max level professions is essentially useless. I expect it will be useful to have every profession at max in Shadowlands as well, as this has typically been the case in every expansion, so leveling to max to get them all can be well worth it. After that though, there is still value to leveling more alts, but primarily for old expansion items.
Useful lower level daily cooldowns
There are mainly three professions that have lower level daily cooldowns that still remain relevant. Alchemy has access to transmutes, Engineering can craft Jard’s peculiar energy source for the Sky Golem, and Tailors can craft various special cloths and Hexweave Cloth for bags.
Overall I think Alchemy and engineering is by far the best combination, so I will stick to describing that one.
I have written a full post on this before, but for this case you are mainly looking for one of two transmutes. Either Living Steel or Pyrium Bars. Living Steel is from Pandaria and requires you to be level 90, whereas Pyrium Bar is from Cataclysm and you can get it at 85. Leveling to 90 is not too hard though, so consider pushing from 85 to 90.
Living steel is usually the main choice, primarily due to the synergy with Engineering as we’ll see.
Make sure you grab the quest for transmutation mastery. This gives you a chance to get extra items from transmutes that translates to 20% extra on average. The quest can be picked up from your alchemy trainer and just requires you to turn in 4 Living Steel.
Engineering is perhaps the best one. The Sky Golem mount is still highly in demand, partially as it allows you to gather herbs without dismounting. It requires 30 Jard’s Peculiar Energy source, which is a Bind on Pickup material on a daily cooldown. So you need to run the cooldown for 30 days to get a mount. The mount also requires 30 Living steel, which makes the combination with an alchemist fantastic.
To get this recipe you just have to kill pandaria mobs until the recipe drops. I used to just go to the Vale of Eternal Blossoms and start killing mobs, which would get it in like 10-30 kills.
Level to 90 to take advantage
These recipes will all remain relevant in Shadowlands as it’s all cosmetic. Leveling some extra alts to 90 if you’re bored can help you generate some easy gold on the side for a long time to come, so do take advantage of the current content lull and XP buff!