Hi @Polyrism, welcome to our group! This is a great question that gets asked all the time. First of all, it’s fantastic that you already have experience with solfege. With solfege you already have a huge head start on the work that we do in IFR. I think that if you find it easier to use solfege syllables in the beginning then that’s totally fine. Our biggest priority in the beginning is just to get you in contact with the sounds, hearing them in your mind and picturing where each sound is located in the tonal octave. If you find that it’s more appealing to do this work using the solfege syllables (either because of your app or because you like that they are mono-syllabic), then please feel free to use them.
At some point in the future, though, you will probably find yourself very naturally switching to the tonal numbers in much of your thinking. What the tonal numbers give you is a much simpler and more efficient way of picturing the relationships between the notes. For example, most musicians will have to stop and think for a moment to picture the number of scale degrees between Fa and Ti. But any child can instantly picture the distance from 4 to 7. This advantage gets multiplied when we start stacking intervals to create chords and entire chord progressions.
But this is an entirely personal decision. I would never want my teaching to get twisted into something dogmatic. The much more important principle behind IFR is simply to do all your musical thinking relative to the overall key of the music, because this is how our ear works naturally. I don’t care so much which system of names you prefer to use, as long as you have a single coherent system that allows you to describe the sounds you hear.
So I would just encourage you to keep an open mind and continue thinking about musical notes in whatever way you find most empowering to you personally. Also remember that you are never going to lose either system. The solfege syllables don’t go away just because you learn to express something in tonal numbers. The fact that you can switch between both languages is a great asset that you’ll always have.
I hope I’ve answered your question enough so that you can keep moving forward. If you have any other doubts or questions about it, please don’t hesitate to post more. Welcome to our forum and thanks for your contribution!