Let me offer a simple suggestion.
Do not think about "making money". Money is a means, not an end, and it seems in your situation (small local market) it's not working to solve your problems. Your covenant is too big for the local scene. It's a little as if, in the real world, you were the leader of a large country that got wiped out by a natural disaster, and had 400 million people to take care of. Money is necessary, but by no means sufficient. You can't think of securing food, medicines, housing etc. for 400 million people in 3 months just by "spending money". What are you going to do, order 100 million tons of food on Amazon? Buy 40 million houses on Zillow?
Instead, think about your end, which is to obtain the mundane stuff your covenant needs. So, the first thing to ask yourself is: what does the covenant need? Food (including water)? Stone? Wood? Metals? Animal skins? Craftsmen (which craftsmen)? Basic tools (including weapons and workshops)? Lab equipment (at least the mundane stuff)? Make a list and prioritize.
Then ask yourselves: of this list, how much can you obtain directly via the resources you already have (e.g., it seems you have a sizable workforce)? For example, in terms of food, it seems to me you can probably feed everyone using a fraction of your workforce and a little spontaneous magic, assuming you have enough land as it seems to be the case. First and foremost, you need freshwater (super-easy to get from seawater through Hermetic desalinization). Then, you need cereals and vegetables (easy), and some source of protein like fish (easy, since you are by the sea). Some eggs, dairy, oil, salt are not strictly needed, but without them fare is a bit boring. Alcohol and Spices are certainly not needed, but they sure make life much nicer. Think how you can get each of these with generous applications of non-fatiguing spontaneous magic, or with just a few fatiguing (i.e. potentially botchful) spontaneous castings. You can do a lot given the Art scores listed above.
There will be a few things that you can't get immediately and directly. For example, skilled craftsmen. Possibly metal. Some dyes and inks (useful for making books). List them, and do not ask yourself how much they cost, but where can you get them. In modern terms, think of having to buy an assault rifle: it does not cost that much, but you can't buy it on Amazon. Only once you have an idea of where to get the stuff, think of how to "pay" for it. When I say "pay", I do not necessarily mean "pay with money". For example, you generally cannot "buy" craftsmen. But you may have found a young monk with a fantastic talent for illumination. How do you convince him to join your covenant, or at the very least to teach his craft to one of your covenfolk?
Of course, in some cases, the answer will be: some merchant network is willing to provide me with some of the stuff, as long as it makes some profit. In this case, the same merchant network may be willing to barter what you need for other goods it may sell. The crucial issue is that not everything will be accepted as payments, and certainly not in arbitrary quantities. Plus, you may have to do some triangulations: produce good A, barter it for good B using merchant network 1, barter B for what you really need using merchant network 2. Ultimately, it's up to your troupe to decide how complicated this should get, based on the storytelling opportunities it offers. Keep in mind that Hermetic covenants have a unique advantage: the Mercere trading network.
Now, there are some fairly common goods that you may want internally produce a surplus of, and trade. The fundamental idea is that there should be a) a sufficiently broad, accessible market for them (good luck trading in emeralds) and b) they should be relatively easy to store and transport (good luck trading in hay) and c) they should be obviously easy to produce. Some common options:
Salt, by seawater evaporation.
Pearls (not a broad market, but great to barter for other precious materials): magic makes it easy to find them and collect them.
Easy to preserve foodstuff (e.g. fish, but vegetables too, olive oil, wine, flour, honey, nuts, marzipan, and pasta). With a little magic you can probably produce much more than you consume, and Hermetic drying is fast and easy.
Fine wool, linen, cotton, raw or processed. Again, a little spontaneous magic goes a long way.
Metals, raw or processed. Not clear if they are available on your island, or in the surrounding area, but magic makes it incredibly easy to find them, and to extract them.
Silk: if you know it's secret, it's incredibly valuable, and not that hard to produce with the help of a little spontaneous magic. E.g. transform a sheep so that it produces silk instead of wool: your Bjornaer can probably pull this off spontaneously, though fatigue will be necessary. Or, transform some covenfolk so that they will grow silk hair, ideally at a much faster rate than normal - that's just a few MuCo5 effects! There's not a very large market for it, but still...
High quality stone, like marble. Not too easy to transport, but easy enough to quarry in large quantities with spontaneous magic.
Soap, like the famous soap of Aleppo.
Apparently valuable stuff that's just an illusion. Dangerous, but you can easily pull one big sting: offer your clueless mudane merchant network a very valuable cargo, something worth a few years of your provisions, and arrange for some "accident" so it gets lost at sea...
Let me just look at the first one as an example. Salt. One cubic pace of seawater provides about 20Kg of salt. How much that might be worth varies widely, but putting it on par with wheat or maybe a little less (say, half as much) is reasonable. So, three Individuals of sea water provide the equivalent of a bushel of wheat. The merchant network needs to profit too, so, I'd say that you can exchange the salt of 10 Ind for the equivalent of bushel of wheat, or, roughly, salt from ... 500 Ind of seawater for 1 Mythic pound? Something like that - the troupe will have to decide the exact ratio, but you'll note changing it by a factor 2 or even 4 won't change things much. Exercise left for the reader: how much can one of your magi "earn" by spending a single day (8 hours) engaged in non-fatiguing, spontaneous spellcasting extracting salt from seawater? Show your work, including the spells!