I had a similar thought recently; I'd gladly pay 15% to someone that can consistently find me interesting work that pays well.
We pay 17%. So not quite the 15% you mentioned, but also far from the 30% others talked about.
We're only in Sweden for now, so maybe this isn't immediately accessible to you. But it shows that it can be done.
I have talked to consulting/contracting shops here in the US before, which might look superficially similar, but there you are generally a direct employee of the company who then contracts you out to their clients. I have generally had a poor perception of such places, though I'm sure some are fine. Kvadrat sounds much better!
My overhead for cold-finding new clients is <5% (against those projects), and I'm not a good salesman: I'm overly direct, opinionated, and have very low tolerance for nonsense. My overhead for recurring clients and referrals is <1% (against those projects), which is also the majority of my business.
Admin, billing, accounting, etc is also <1%, so it's a non issue.
These sales organisations have _much_ better salesmen than me, yet they need >3x the cost to do the job, and they produce worse results?
My guess is: if you actually tried to spend one day per week (20%) for sales and networking you will get a better result. But for many (including me) it is uninteresting / uncomfortable / annoying / etc and therefore many of us overestimate the difficulty of sales.
There is also very likely a notable difference in interest alignment between the sales partners/employees and you. They will make more money/h the faster they sell you and therefore have greatly reduced monetary interest in working significantly more for a higher quote or a more specialised project. They also do not have the deep domain knowledge you do in your areas, and you are much better at judging how well you will enjoy working with a prospective organisation and how interesting you will find any given project.
My experience: Yes they can sell you. They will get lower quotes than you will get yourself. They will find worse projects than you will find on your own. And they cost too much. Still, I had fun with all the groups I've tried, and I still recommend them as a social luxury. Fun, overpriced, business friends.
Being a freelancer myself, I've often asked recruiters about this, and they always dodge the question or reply something like "there's much more to our job than you think". I have not yet figured out what that is exactly.
My best guess, from personal experience, is that they charge 15% and upwards because... they can. Because a lot of (especially technical) contractors are really bad negotiators and just accept whatever is offered.
Given the huge competition between recruiting agencies nowadays (at least where I live in Europe, it seems like there's 10 recruiters for every senior developer), I would think that a new recruiting agency could gain a huge competitive advantage by simply advertising clear & fair commissions.
Instead, all my cold calling, contract negotiations, billing and accounts receivable — things I'm not good at and don't enjoy doing — get taken care of. I'm happy to pay for that. The "luxury" of "business friends" is included.
If the gigs found are high paying enough I don't think 30% is unreasonable.
However, I'd be fine with any kind of work, it doesn't have to be firmware.
In my previous position we built a lot of wearable devices so having a dev system at home was no problem since they're portable by definition. Likewise at my current position, we have larger, but still easily movable development/debug systems since the product I'm working on fits on a desktop. Takes up more room with everything splayed out on a back plate, but still fits on a tabletop.
But going back two jobs where the machines we built were large, very complex and floorstanding, it would be much more difficult.
It's also much easier when the hardware is stable. When you're still in the "bring up" phase, there's a lot of shipping hardware back and forth, or occasional office days together if feasible, especially if you need a $70,000 tool that the company doesn't want you taking home.
As the pandemic has progressed, embedded development has adapted. The biggest reason it's likely to reverse is simply that a lot of companies prefer to have their people onsite.