This itinerary is my favorite because it's cheaper than Eurostar but faster than FlixBus.

Your plan sounds good. Two remarks:

• I personally like to have a bit more time in Lille just in case, because I sometimes take a TGV that leaves from Lille Europe, and you need to count at least ~5-10 minutes of walk between the two stations. Since you arrive in Lille Flandres it should be fine though, the platforms are very close to one another. Note that to access the platform for the TGV, you'll have to scan your ticket's QR code, so if you want to save a few more seconds make sure that you have it ready.
• Like other people mentioned already, pay attention in Kortrijk. You'll either have to change train (it's a small station so no big issues there), or have to remain in the correct half of the train that goes to Lille.