Here's a comp for Felix: Richard Gasquet. Both showed up on the radar at a young age. Year-end rankings through age 24:
RG: 161, 93, 107, 16, 18, 8, 25, 52
FA: 601, 162, 108, 21, 21, 11, 6, 29, 29
Sort of a similar pattern: both made a big jump - RG from 19 to 20, FA 18 to 19. Both had their best ranking year (so far) at age 22. Both fell back to the 25+ range after.
Gasquet rose again in his late 20s and remained in the top 30 or so into his early 30s, and has been more in the 50-80ish range since. Who knows how FA might turn out, but I could see him eventually having another spike, maybe ala Dimitrov (another somewhat similar player, in terms of possible career pattern and level).