The difficulty with Duolingo (free tier) is that it's so limited in how much you can learn per day. I have a streak of about 600 days and I'm only about a third of the way through the total program. I average about one lesson a day (5-10 min); in the beginning I was able to do a lot more, but now I will not-infrequently fail my first lesson of the day due to difficulty, and have to do a "practice" lesson (which are too-easy review lessons that don't require "hearts") just to keep the streak alive.
I also didn't even realize until relatively recently that you could move on from one lesson after completing the first level, so I have apparently been inefficiently learning each lesson to level five this whole time. My recall of stuff from the beginning of the program is already faded...
Maybe it's better if you pay for it just to be able to do more lessons, or alternatively maybe the desktop application described in the article is still active and doesn't require payment?
The practice lessons give you back hearts, 1 per practice plus another 1 if you watch a short ad afterwards. Starting a new lesson will sometimes also give you an opportunity to watch another ad for a new heart, so after doing 2-5 practice lessons you can go back to progressing the new material you were working on before.
I actually prefer the free version, because it naturally creates spaced repetition by forcing me to review old lessons in-between new content.
Duolingo was more useful to me when you could fail until you got it right instead of losing your hearts and then getting bumped out of the lesson.
I don't use it any more, but when they added the heart feature, I finally bought a subscription. Without it, Duolingo ends the lesson just as you're about to learn from your mistakes which is unfortunate.
The hearts feature also encourages you to repeat the lessons more. It can be more tedious, but may be better.
Also with the mobile apps there are these special XP challenges which you can use to repeat what you have learned before. I use these a lot, and it's more fun. I also notice when I'm too far in front of my skis, in that those exercises seem to difficult for the time allotted. That is a sign that I progressed too quickly through the lessons.
Anything else might be a waste of time, including conjugation apps. The exception may be actual grammar references which can sometimes be helpful.