After the previous failure, I decided to document myself at a minimum. I mainly used this article from Rishikesh Daoo. I might consider reading some of his other articles as it’s quite insightful!
This basically told me that a nice ratio can be reached with 4 buffers that delay at different rates the sound.
So while I kept the previous algorithm, I just duplicated it 4 times with different offsets to the main delay offset. So instead of having only 1x repetition fading progressively, we have 4 of them:

By adjusting the delays time, we can get an “acceptable reverb effect” that covers very different types of reverb:

When the delay time is just under the signal frequency, we get a reversed delay (like if the reverb was applied from right to left). This transforms the shape of the resulting sound (as it reduces the “saw” effect):

I’m quite happy with this result, it can be found in the blog/002-reverb-effect branch of my project.
Next, I will probably add a UI so that testing sound can be more interactive. This is important as my aim is not to build a synth following the theory only, but more to explore sound design while building my synth. Soon too, I will need an envelope (and so a Voltage Control Oscillator) and midi assignments… Not sure in which order this will happen 😀
Until then, stay safe and curious!