Logically speaking when you add to the workshop quota what you're doing is actually performing the action to X part. So for a blacksmith it's not just you making the materials but then also using your skill to attach said materials to what you're currently making.
I think needing 3 others is a necessary evil as it prevents people who buy solo houses from just having a solo FC and owning a FC house and having the FC workshop all to themselves. If it was like that it be much harder to tell the difference between a solo home owner and one who had 1 friend help them invite their mules to their faux FC so they could use the workshop for themselves (on top of the 4 airships they would also have access to).
Rather than ask them to remove the need for 3 others we should focus on a way to allow the 3 other members to have an influence over the results of the recipe.
There could be mini-games introduced that are completely optional but have the opportunity to improve the % in which you can save materials. Designate 1 person as the handing them in and the other 3 perform task on the proper class to use the materials in question efficiently, if they reach 0 with the materials then they don't gain back any of the materials but if they manage to complete the task with leftover materials they are put in a pool that can be retrieved once the recipe is finished or cancelled.
I think if you don't want your friends to be bored, rather than find a way to turn workshops into a complete single player experience, come up with ways they could make it more interactive.
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One more edit.
There are 8 crafts in the game, as it stands Culinary does get the shaft when it comes to the current workshop items. What SE could actually do is improve the interaction with the workshop by allowing a full party of 8 the opportunity to be the most efficient with their materials as possible.
First change I would make it have it where the workshop now takes items out of the FC chest rather than needing them to be input manually, this would save a lot of the tedious interaction since as it stands now the xp gains are pretty worthless.
Second when you engage in a recipe it confirms that you have all the required mats before executing it however before you execute the craft it confirms each person's current class and commits it for the process.
Then the mini-game commence, each class of the proper type works to make X material efficient to save materials or even double the quantity of the current recipe.
Basically the process could come down to either regaining 1/3 materials or creating 2 of X item in one sitting. Maybe even HQ or HQ+
With Culinary they would have the unique game-play aspect and maybe Alchemy as well, offhand I can't recall how many recipes require alchemy but basically Culinary could have unique foods that work for workshops only that give your allies an edge when being efficient with materials, while alchemist could have special vials that work to increase the quality of the materials being used.
What hurts workshops the most is lack of real control over the outcome of each step, even 100/100 doesn't guarantee a better progress result so you're at a point where it's like there is 0 point in making HQ because it's all a crap shoot when it comes to saving on materials.
Workshops need a restructuring to give them more interaction, not less.