I would say 18% is worth it for your time, peace of mind, and knowing it's getting done right and him being the bad guy to make the subs come back and finish stuff or fix stuff that they inevitably won't do or will do wrong.

However, and this is a big however. The GC doesn't have any incentive to do the project on the cheap, since it's not coming out of his pocket and he's making a cut off the whole job. It's more beneficial for him if your shop costs $300k versus $250k.

You, of course, will be much more cost conscious.

If you can get a whole job estimate and feel confident that he can stay within that number that works for your budget, use a GC.

With using a GC, you'll make progress payments and you can withhold payments if the subs haven't finished or if they did something wrong. It's up to the GC to get them to come back and finish. If you're dealing directly with the subs, you may run into one or two who get to the point that just stop answering your calls, take forever to show up, and possibly disappear altogether halfway through the job. You'll have a harder time getting subs to come work without a down payment, then you cross your fingers they show up.

It doesn't sound like a super complicated build. You could probably do it yourself. Depends on building codes, the complexity of the engineering, and a bunch of other factors. Just be warned. If you do it, it will take much longer than you think and still cost more than you hope. But will probably end up cheaper than using a GC.

You're essentially trading money for peace of mind and time using a GC.