I am not saying that we at Turris are doing a great job here, but I haven't seen many manufacturers to contribute to OpenWrt as we do. You can try to convince me, but before that, let me show some statistics:
On GitHub, I opened over 100 pull requests, but it is not possible to count only GitHub pull requests as I sent a few patches via the mailing list.
Not sure, if you count the packages repository as well. I opened over 500 PRs there.
But that's only me, you need to count other guys from Turris as well. Sadly we are not unified on this thing, so it is somehow problematic to find it out, but you can check it e.g. here.
This is only related to OpenWrt repositories, but the main development field is (and should be for everyone) the Linux kernel as everyone can benefit from it. They can use any GNU/Linux distributions, which they want. Our kernel developer improved support for SoC A3720 as much as he could, although we don't get any thank you from other manufacturers.
You tried to keep it as short as possible, right? Because it sounds to me that you put it very modestly.
Turris OS is based on top of each OpenWrt release, which helps us in many ways in development. Unfortunately, what slows us down is that we can not add new features like U-boot support or keep it updated in the stable releases and you need to add that OpenWrt do releases once per year.
Also, OpenWrt tries to be minimalistic to support as many devices as possible, and that's one difference between Turris OS. We did not do such a trade-off in size, so we can focus on speed. Also, there are some things that we can not upstream as automatic updates, our UI side-by-side with LuCI, etc.
Well, I could add more things, what is different, but I want to highlight one issue, which is now happening in OpenWrt 22.03 release, is the DSA/switch issue on mvebu. There were two options - look into it further, backport many changes in the DSA subsystem from a newer Linux kernel or use the latest LTS kernel. That's why we have LTS kernel 5.15 for all OpenWrt versions.
Anyway, what I am thinking is that it would help manufacturers not only to us but be more in OpenWrt meetings or even be part of the core team. Because from my point of view, every developer has different opinions and sometimes you can things merged, and sometimes you don't. I am aware that things can be broken when you are backporting stuff to the stable release, but sometimes you don't have a choice. You can not ship OpenWrt RC versions or even daily snapshots to end users.
EDIT: Oh, I almost to forgot to mention that we sent a few routers to main OpenWrt developers and we are willing to continue with that.