If those are your only journeys, regular tickets might be cheaper. Otherwise a 4-day global pass (even if you only use 2 days). You need to add seat reservations on top of the pass :

- Eurostar 30€ (limited quota, can sell out weeks in advance)

- TGV Lille/Paris - Nice limited 10€, then 20€

- TGV Nice - Paris limited 10€, then 20€

With that I assume that regular tickets will be cheaper but it depends on your travel dates

Simple maths will usually show that a journey like this will in fact be better with a fully flexible Global Pass once you factor in the flexibility of the pass allowing unlimited travel on each day against the inflexible advance purchase tickets.

Choosing a random date of July 12 2023 I would travel on this routing:

Lon - Paris E* 0601 - 0920  Advance purchase £74     Pass reservation £30

Paris - Nice 1122 - 1658  Advance purchase 142 euro Global pass max 20 euro

Assuming similar prices on return that means the return with low flexibility fares would be about 450 euro.

A 4 day Global Pass would cost 258 euro plus a max of 100 euro reservation fees 358 euro) AND allow you a further 2 days travel along the Cote D’Azur. 

Yes you could travel other days and less attractive travel times and save a few euros but I cannot see how you could travel Lon - Nice by any combination AND 2 days free travel for less than 358 euros.

Yes you could travel on the cheap Ouigo services or regional trains in France but these are less luxurious than OUIGO TGVs.

A first Class pass would still be no more expensive than the second class advance purchase.