Island: I don't really get workshop crafting/trading...

I mean i do understand it.
Make item there's insufficient stock of and high popularity and profit..... at least that's what they tell us.

I however have noticed something frustrating.

Today Silver ear cuff's showed as insufficient stock and high popularity so i set them up to be crafted.
They take 8 hours to craft.
Just before they finished crafting i checked on them.
The demand? In that 8 hours they had changed too Sufficient with plummeting demand and popularity.
I made pretty much next to nothing off them and wasted 8 hours of my workshops time based off the information the in game guide gave me!

So am i missing something? Because I'm not quite seeing how this is supposed to work if the values shift to the opposite extreme half way through the time it takes to craft them.
The guide makes it out to be that, if you start crafting an in demand item with low stock, it will be that way when you finish so you profit off it, but if the demand can vanish due to RNG or whatever, it makes that demand/stock guide totally worthless.

Am i missing something here? As the only source of Seafarer's Cowries id expect at least a fair chance of being able to make them without RNG resulting in my craft's becoming worthless before they are even finished crafting, and its not like we can hold onto our crafted items until prices change back in our favor, they are automatically sold!

You're not alone.
Popularity changes weekly - you can have a look once and then set up your workshop for whole week
Demand changes without reason as you mentioned already - doesn't make sense to look at demand at all.
I like Sanctuary Island... but the popularity/demand/in stock window seems to be buggy or looks difficult without beeing worth to look at it.

It is not the only source of blue cowries tho. The rare items your granary brings you can be sold for 25 cowries each and the mats your animals drop can be also sold for blue cowries! Same with the crops from your field. If you add that all up it can bring you also like 500 cowries each day easily. But ofc it is better to use these raw mats in your workshops.

That said, just chose what is in high demand and low in stock. Produce things of the same category. Like make a weapon after a weapon and they are also metalworks so make a bronze sheep after that which are also metalworks and the output will be doubled through these combo chains.

Also build your workshops to level 3 as soon as possible and build the decorative buildings, these also give a bonus on your mammets motivation which also increases the cowrie output by a lot!

Also try to start always with a 4hour craft and then do the big once, keep the best of your crafts for the end of the season when the mammet motivation is maxed out. If you have enough mats ofc you can just do the mats expensive crafts all day.

I just spam whatever and wait for it to be crafted.

I just do not want my mammets to cry...