Personally, as a free software enthusiast, I feel entitled to the "four freedoms" with every tool I use, no matter how large or small. I have no problem paying for free software and have done so in the past. For me the four freedoms are the point. A proprietary license is either a dealbreaker or a very large downside.

So, for me:

Is source‑available meaningfully different from closed‑source?

No. "Source available" in this context is a type of proprietary license. The fact that source code is visible does not make it not proprietary, because it is shared under a license that favors the interest of the rightsholder above those of its users. I talk about this often when contrasting so-called business and ethical licenses with true FOSS licenses. A true FOSS license grants modification and distribution rights and does not impose usage restrictions, a proprietary license imposes usage restrictions. With a FOSS license I don't need to worry that whatever I'm using the software for somehow infringes the rightsholder's personal ethics, and it encourages forking and code reuse.

In other words, thinking about it in terms of whether the source code is open or closed or "available" is missing the point entirely for the free software community. The point is what are you are allowed to do with the software and what restrictions are the rightsholder imposing on your usage of the software. Keep in mind most users are not programmers and thus being able to see source code does not impart any direct advantage to them, but allowing the community (which does include programmers) the four freedoms means things like forks and customizations can be spread.

Do you expect small tools to default to open‑source?

As said above I use free software wherever possible. Thinking about it I guess I generally do expect a small hobbyist tool (as opposed to something that exists to be a product) to be free software, but then again I use platforms that are favored by free software enthusiasts. On Windows I suppose it's more common to see these as proprietary freeware apps.

Does hosting something on GitHub imply a FOSS expectation?

For me it does not, I've learned to always look for a license to make sure, but I think a lot of people do not understand that GitHub can host proprietary projects too.

For someone planning a larger ecosystem later, which model is the most reasonable starting point?

You'll need to elaborate on this more. If you are planning to grow a free software community then using a true free software license is important. Free software and open source licenses are known to not impose usage restrictions that favor the rightsholder's interests above the user's, as I have said. On the other hand, if your goal is to create a business around the project, you need to balance your users rights against your business interests. Starting out with a free software license then switching to a proprietary source available license once you have a captive ecosystem will create resentment and guarantee a community based fork of the last true FOSS version.