The answer is no. And it's the horrible consequence of DLC.

We used to have expansions. Which required the base game, but were sold separately.

DLCs were panned across the board when they started, for offering a questionable worth of content for small but significant amouts of money.

And then DLCs were integrated into platforms like Steam.

And then Steam's functionality of providing evidence of ownership made expansions "easier".

I am not qualified to describe the entire history/transition, but nowadays DLC weakly implies rights restriction. Whether that is DRM or not depends on the extent and marketing.

After owning Rimworld on GOG, and buying the DLC: Biotech, I refunded Biotech here and subsequently bought (Rimworld+Royalty+Biotech) on Steam because due to regional pricing, those three were cheaper than (Royalty and Biotech) on GOG.

Ideally I would have just bought Royalty on Steam, and installed it to my existing stack. But it doesn't work because you need the base game to buy DLC anywhere.

If that is what you think, then you have it wrong. GOG is NOT a client-based game provider like Steam. You do not need a client at GOG, and aside from some same games, the stores are not really alike. GOG is not an alternative to Steam ... except in a very very limited sense. Steam offers far more than GOG does or is ever likely to. That said, my personal view, is that GOG is superior, which for me is based on DRM-Free installers, that you can even download via your web browser or Galaxy the GOG client or via a few third party options such as gogrepo.py or gogcli etc. If you solely buy your games from GOG, then as a gamer you will severely limit what games and features you have access to. That doesn't matter enough to many of us at GOG, but it does to many others and perhaps even you. That said, you can use both GOG and Steam etc.

In theory, if you could buy DLCs on their own at GOG, they would work with the game bought at another store, but some form of DRM is likely to prevent that, though I have read of a few successful cases, from folks that own the base game at more than one store.