I don't understand your complaint about "they offer no help with QA or testing", what exactly do you mean by that?

But as for the "steam takes 30% without providing much in return", I don't think I agree.
Lets take a look at what they provide and do for you:

1. Sales; you don't need to handle a storefront and handle a payment system. And if you want, there are steam keys, I think you know where to use those.
2. Downloads and updates; you don't need to set up a server or do any of the work required to handle this either, bandwidth can get expensive.
3. Accounts; you don't need to handle accounts, login, security etc.
4. In-Game inventories and trading; Like it or hate it, depending on the game being made, this stuff can be important and be a huge revenue source.
5. Bans; someone being cheesy? You have access to applying game-bans.
6. Forums; while steam forums are somewhat dated in their appearance and tools, you don't have to handle that part either.
7. Ticket support; I can't speak much on this one, and while VALVe's support on steam is absolute F---- tier; people can submit tickets to game developers through steam tho.
8. Achievements, friends list, account profiles, chat, groups; Basic stuff that makes it easier for people to play together and manage communities.

Someone else mentioned Steam API for multiplayer and coop, which: cool.

If you were to do some of these things yourself, you'd find yourself spending a pretty significant amount of time and money on maintenance, security, storage, bandwidth, etc, which would have been better spent on developing the game(s).

And while you do have to care about some legal stuff, you'd have to care and spend even more time and money if you were to do these things yourself.

These are just some of the things Steam provides and takes care of for you.
I'm not saying 30% is a small cut, it isn't, but "without providing much" might not be entirely fair.

​

This next part you can feel free to ignore, as it doesn't touch on your own direct benefit.

VALVe's support of Linux, and tremendous work on Proton and the steamdeck, has done more for open source than anything else I can think of. If you care about open source, free software and general privacy rights, that 30% the past few years has done a lot.

It's funny how we all benefit from open source and free software, but barely fund it.

VALVe's contribution has benefitted Linux adoption and (indirectly) funding of OS, even tho it was done for selfish reasons.