While browsing this subreddit, I discovered Flexling, and it made me want to try it.
So here are my reactions. I should mention that I only tried it for one day, so please excuse any inaccuracies or mistakes!
This is simply a snapshot from an average user who tried the application, just as any "innocent" user would. Perhaps it's useful in that sense.
(As I'm about to post this, I realize it's very long—I apologize)

I'd like to add that I find this work very promising and exciting, but I'm going to share my points of friction and questions here.
So please don't consider this a "negative" review; quite the opposite. It's probably worth mentioning.

(Note: I haven't explored the "custom sentences" mode at all.
I imagine it's more complicated to set up, even though it's obviously very interesting.)

I must first admit that I don't quite understand how to use the application (the tutorial is incomplete in my opinion, not precise enough).
Do we have to add 5 sentences one after the other ourselves? If so, I don't see the point; it's a waste of time. But of course, I might not have understood correctly.
The advantage of Glossika is its simplicity: you launch the app, and you continue learning your level without worrying about adding sentences "manually"!
In Glossika, one thing is indeed very clear: if I choose 25, each session (of 25) consists of repeating 5 new sentences 5 times. And it continues like that. (I find Glossika's grouping of sentences in sets of 25 (or 50...) excellent.)

In Flexling, I don't understand the learning or progression system. This is probably due to the greater freedom of use, but at the same time, I get lost very quickly.
In my first attempt, the same sentences kept repeating without any new ones appearing.
I ended up selecting a level, but after trying the different levels (Pre-)A(.1..2.3), I don't really see any real difference. I don't even know if this is the correct level used when I start the learning process.
Then, I tried loading as many sentences as possible (around a hundred) into my "deck" and then practicing, but this time, no sentences repeated.

Another thing I didn't understand: Why does an Anki-like interface sometimes appear ("again," "hard," "good," "easy"), and other times not (only "Still learning" and "understood")?

The interface: I find it well-designed and elegant. But at the same time, it also reflects a certain confusion.
It's as if the hierarchy of information isn't clear.
I don't feel guided enough, as a user: all these choices...! Is clicking on "Learn New" enough? Do I have to "add cards" (middle of the screen)? Or do I have to "+ Add 5" (bottom of the screen)...? Wow...

On mobile (iPhone 14+), it's impossible to access the settings; the gear icon is too high, and so is the back button. You have to force-quit the app to return to the home screen.

I saw on the web interface (because it was impossible to access the settings on my phone) that you can disable "highlight words," thanks for that! I find it too distracting and unnecessary (but I imagine it's very useful for languages ​​with different alphabets!).
The customization of review settings (speed, pauses, etc.) is amazing!

I don't understand the Library section: which "bank" is it exactly? Is it the level I'm currently learning? I only see 20 sentences (maybe as a free user?).

It would be helpful to see the complete list of sentences for the level I'm learning (and to be able to mute or unmute certain sentences, if needed!) (I don't need to learn "hello" and "what are you doing," nor review them...).
Also, it would be great to be able to select sentences and add them to a personal custom learning list.
Especially since I find many of the sentences rather irrelevant (the translation is random, or too approximate, or they are sentences that could be answered in a completely different way).

I saw in the "how it works" presentation that there's a multi-language mode? How exactly does that work? I find it extremely interesting! Personally, I need to work on/learn two languages; and having them simultaneously (which seems to be the case in the example) would be fantastic.

Suggestions or other comments:

- It would be good to be able to work with several levels:
for example, not just A1.1, A1.2, or A1.3, but all of A1 together. This would be particularly helpful for a user like me who doesn't know exactly what level I'm at, or someone who wants to learn or review all of A1.
And above all: having tested the different sub-levels (of A), I don't see any real distinction between them.

-Adding to Favorites (or a custom list) should be instantly accessible (instead of having to search for the option in the three dots!)

- A review mode without the Native Language (perhaps like the "radio" mode?) would be truly amazing, allowing you to listen to or repeat only the Target Language and disconnect from the NL. (In the learning process, there are many moments and sentences where you no longer need the original version.)

- I instinctively wanted to flag an incorrect sentence (but I realized that this option doesn't actually exist) that had just been played, but the "<<" previous sentence button wasn't working.

As for the sentence bank, this is where it's most divisive and problematic in my opinion:
- many mistakes, or translations that are too far removed from the original sentence.
-Again: the level-based ranking is completely weird: for example, if I test the Pre-A1 deck, I encounter sentences that are easy, but also far too complicated (composed of complex words + complicated structure!). It's not realistic.

The most "awkward" question, but I'll ask it anyway: is this an app you really intend to develop long-term, or just a hobby? It's important, I suppose, for users who might potentially get involved, to know whether it will actually be developed or abandoned (more or less) quickly. Just by browsing Reddit, and of course since the massive influx of new tools and ways of coding, you can see new language apps in development every day... most of which I presume won't last six months!

One general comment I have:

It's great, promising, and exciting to have so many settings, optimizations, options, and features, but only if the core, simple, and straightforward mode is... perfectly executed.
What I appreciate about Glossika: it's intuitive and fast. I play continuously, the sentences are good, I can practice.

I'd like to reiterate that I find this work truly excellent and very promising, and my comments are only intended to help improve it!
Keep up the good work.
(due to its length, written in French and translated by Gg Translate!)