Perdonally I've heard a ton of good things about Language Transfer despite not using it myself

Duolingo does have that function. It's much more obvious on desktop web, but in the phone app, you tap the notebook icons to the right of the headings. I mean, they're not necessarily excellent explanations, but they're there.

The old tree had grammar bits (for some languages only in the browser) but the new one is just bad

Lingodeer is a technical mess with popups, banners and lock or crown icons everywhere. There are situations where it just won't let you continue to the next lesson and the flow inside the exercises is very janky. Turning off the animations helps a lot but it's nowhere near the ease of use of Duolingo.

The problem with all these alternatives is that the language selection is extremely limited. You want to learn English, French, German, or Spanish? Great, there are a million options for you! But if you go a bit more niche like Finnish or Irish, your options are much more limited. Of course there are ways to learn those languages - and much better ways than Duolingo. But Duolingo's strength is offering a bunch of them, for free, in one place.

Note that I'm not trying to defend Duolingo, but rather deploring the lack of alternatives.

Just gonna hijack this and recommend https://www.renshuu.org/ for everybody trying to learn Japanese.

It's probably the best compaion-app I have found for learning the language.

I use Duolingo (and actually pay for it) and I agree 100% with this. The app is primarily about keeping you engaged and on the app. The method is by attempting to teach you a language.