Feedback on my language learning plan

Hello everyone! I'm about to embark on a language learning journey but I wanted some perspective and feedback from people more advanced in their journey than I am.

I'm portuguese and I can also speak english at somewhere near native and I understand spanish pretty much perfectly (though I'm not confident in speaking it).

My language knowledge so far as been automatic and a result of that child like brute forcing of learning, so I don't have much experience with more formal language learning. I did give it a try with japanese but I was too young and basically only did Duolingo + Anki which ended up being very annoying and I eventually just gave up.

My plan right now is to try to learn french as sort of a "tutorial" and as a way to workshop a language learning process that is fun and engaging for me before moving on to mandarin which is my ultimate goal.

Both have pretty complex reasons as to why I want to learn them, but I wanted to reflect on why my previous japanese journey didn't work and this is where a different, more mature perspective is very valuable.

I did have a lot of motivation, but I felt like it got progressively harder and very repetitive, and I stopped feeling like I was making progress. Even when I dedicated a lot of time, it just felt like I was practicing the same stuff and like I was not really consolidating the new vocab. It made me feel dumb and undisciplined, but as I've come to understand, this usually just means a flawed process, and not a display of my shortcomings (especially knowing how much people in this space flame duolingo lol).

My plan is to use a more diverse set of tools, with different aspects to it (listening, conversing, less gamey type of apps) to try to have a more dynamic learning experience, hoping that these different aspects will synergize, but I'm very afraid of plateaus and I wanted to know what did you guys usually do when you found yourselves in one as it is probably the most challenging part of language learning.

This and any another advice/perspective would be appreciated :), thank you for your time reading my post