So all that's left is B-sides; 2 strawberries in level 8, and level 9.
Once you beat Core B (which requires 15 hearts to enter, so you're forced to do the previous B-sides first), you unlock C sides for the first 8 chapters. Don't worry, they're very short.
Finishing Core B also unlocks Golden Strawberries, which want you to beat a chapter without dying once. The early chapters aren't too bad, but the later chapters are really hard. Unless you have a ton of time, I recommend not doing them.
If you beat Forsaken City A without dashing, you'll get an extra strawberry. If you don't know what you're doing though, it's basically impossible. So just look it up if you're curious.
If you want some details about Farewell:
-About halfway through, there's a heart gate (like the one in Core) that wants 15 hearts. So just do the B sides first. (C sides aren't necessary.)
-Despite what the journal says, there is 1 strawberry in this level (not including the golden one).
-There's no B or C side, it's just one long chapter. And boy is it a long one.
-There will be a lot of difficult puzzles and movements, but you already knew that; it's Celeste.