As a rule a rail pass does not work out cheaper if you travel a single travel in a single direction. Your side step to Landerneau can make it worth the money if you can only buy your tickets on the day. But if you can book early, you will likely find good deals.
The pass will also cover your side trips, but you may need to pay a reservation fee for the French bit as well as for all the other fast trains.
There are the offers for reductions when you have the pass but often you will find the same reductions for other reasons, or you will find you can not benefit from those reductions for other reasons. (Like being in the wrong time of the year for the offer.) If you have the pass, you should look to get reductions with it, but it should not figure much or at all in your financial plans when you compare prices.
In the Netherlands and Belgium you will always find train ticket prices to compete with the 'one day on your pass' value.
For the fast trains into and in France you can get the best deals if you book early, but even when you book only a few days in advance you can get tickets for good prices.
I had a look just now and in three days you can travel Brussels Midi to Paris Gare du Nord for €55 although most tickets for that day start at about €99. You can find the prices on one of the French tickets selling sites, this is just one and in English.
Whether your city card in Amsterdam makes financial sense depends on whether you want to go to the museums included in the deal and how much use you want to make of the local public transport.
Again, do your sums, add up the entry tickets and the number of one time (or one day) tickets you would buy.
In many cases it will be near the same amount but a lot of people end up doing less than they had planned when they are without the pass and do a bit more than they had planned with the pass as it is cheap to do that extra museum for just a short visit. Or to take that tram or bus as you have that pass.