Yeah, you are absolutely right. But isn’t the TER an option for you? That one you can take for free from Bellegarde to Geneva with Interrail..

Few years ago reservations from Switzerland to France for TGV were way cheaper (I think around 10 euro or maybe even a bit less). Nowadays they are indeed ridiculously expensive, so I try to avoid using them. Either by taking the TGV/ICE from Germany to France (13 euro, 2nd class) or by going to the first station after the Swiss border by regional train and then taking the TGV with domestic reservation (10 euro).

Yes, I can take the regional train, (TER) from Bellegarde - Geneva.
But it is really outrageous that TGV LYRIA makes it so expensive, for a ride of +- 30 minutes. This while SNCF, as well as TGV LYRIA advertise to use the train for the environment ... and it would be faster than the plane.

SNCF / NMBS also charges an extra cost of 20 euros (2nd class) and 30 euros (1st class) for their TGV Brussels - France. So also for the short ride from Brussels to Lille.
That is also quite a lot, but not comparable with the 57 euros that TGV LYRIA asks in 1st class.

Well, yeah, I mean I agree with you. But they also want to earn money (and have to), and with pass users maybe the revenue is lower than with normal tickets, that’s why they limit it or make it unattractive to use (unfortunately).

There are quite some routes, where it is really annoying that you should pay a high reservation fee for just a short ride… but rules are rules.

The advantage of this: with avoiding the high-speed trains you often see the more scenic routes to see it in some positive way also :D

And actually it is not just about Interrail: I remember paying ridiculously high fees for the short ride across the border from Arlon to Luxembourg (maybe like 8 euro for 15mins), while in Belgium itself I was paying like 5-6 euro per trip (for 3 hours e.g.)… so in general the borders are still a problem with tariffs :(

From Belgium I see 2 options that are the cheapest.

1/ Denderleeuw - Lille Flandres (IC via Kortrijk), or via Brussels.
Lille - Lyon ( TGV )
Lyon - Geneva (TER)
Geneva - Nyon
Nyon - La Cure

2/ Brussels - Frankfurt main
Frankfurt main - Basel SBB
Basel SBB - Bienne
Bienne - Morges
Mornings - Nyon
Nyon - La Cure

3/ If you travel via Cologne, from Brussels, the Eurocity is offered from Cologne to Bern.
Also no seat reservation required.

With Interrail Global Pass :

1/ = 10 euros for TGV (1st and 2nd class)

2/ = No reservations needed