On august the 3rd we are moving from Paris to Florence. I found a ticket wich costs 76E and runs during the night.
Actually 76 EUR isn't that bad of a price for the summer and such popular route, thus it will be hard to find a better fare. Cheapest 6 berth couchette reservation for direct overnight train (Artesia de Nuit #227) cost 25 EUR, thus this is the minimal additional price you will have to pay extra with pass.
On august the 5th we are moving from Florence to Rome. I found tickets starting from 30E but there were also some tickets from 45 E in trains that took much less to do this trip.
30 and 45 EUR are the full fares for fast and high-speed trains on the route. But you can actually travel for 17.05 EUR on regional trains (marked as Regionale Veloc on TrenItalia website), this fare is standard, thus will not change. Also special discounted fare (called "MINI" offer) available for fast and high-speed trains, for August you can actually by tickets using this discount.
For regional trains on the route you don't need seat reservation, for InterCity trains it cost additional 3 EUR, while for EuroStar - 10 EUR.
On august 11th we are moving from Rome to Lyon (we are visiting the city of Ars). I found many many tickets for a night trip. One of them was for 103E.
Actually for your date tgv-europe.com website offers discounted tickets for 82 EUR (including reservation) with one change in Dijon. The only nuance is that you need to input "your country" as one of the EU states, thus it won't redirect you to agency website.
Since on part Rome-Dijon it the same Rome-Paris Artesia de Nuit train, then reservation for pass holders will be the same - 25 EUR for 6 berth couchette.
On august 13th we are moving from Lyon to Lourdes.
Here it's a bit problematic to travel overnight, but on the contrary plenty of choices during the day. Best seems to be to buy two separate tickets one is Lyon-Toulouse (for 13/08 from 37 EUR), the other one - from Toulouse to Lourdes (full standard fare there is 25.7 EUR). But in case you don't like 37 EUR option timing, then one through ticket (app.from 70 to 80 EUR) also will do.
In France you will need to pay extra 3 EUR reservation for each leg with high-speed TGV/Teoz trains, for regional train no reservation needed.
On august 15th we are moving from Lourdes to Madrid (I only found a daylight trip. I understand our only option is to take a train from Lourdes to Hendaye or Irun and from there another train to Madrid). (I don't have a price, but I think this can be 150E).
Actually curve via Irun seems a bit too much in this case, thus it might be easier either to catch direct overnight bus from Pau or Spanish domestic trains run from Canfranc, which is almost first town nearby the border. From there it takes only 6 hours with one change in Zaragoza. Canfranc-Zaragoza ticket actually cost only 12 EUR.
Our schedule is quite tight. We would prefer to have all of our trips confirmed and reserved from our home country.
Well, for most of your legs it's not a problem to buy tickets in advance online. With only reservations (as addition to pass) it's a bit more complicated, but also manageable.
Are there any cheaper/faster options?
In certain cases bus might be cheaper, in certain low-cost plane, but then you need to account overnight stay somewhere etc. Also maybe certain change of sequence of cities to visit might lower total travel expenses.