For the chain tensionner, the original one around the chainring is excellent, it works really well with as consequence that there is no market for a third party solution. Since you are in Paris, no need to buy it online from Pacific Cycles, any Riese & Müller reseller can order it and provide you that tensionner. And I think that Les Vélos Parisiens is still selling the Birdy, they should be able to provide you all needed spare parts.

The Birdy disc front hub is not a standard 100mm OLD disc front hub, the position of the disc is closer to the center of the hub than on a standard front disc hub. The Shutter Precision hub dynamo is a special type made for the Birdy with the mounting for the disc on the right position for the Birdy disc. There are third party Birdy disc front hub available for instance from Hubsmith or from Ridea. But again, you can ask a Riese & Müller to order such a front hub which is mounted on the Birdy without hub dynamo.

For the rear rack, the folding rear rack is excellent, it doesn't modify the folded size. There are third party fixed rear rack but they increase the folded size.

The red rear suspension block is OK for your weight, you do not need the green harder or yellow softer block.

For the front suspension, there are two hardness of springs, I recommend the hard one with 8.5 coils (the softer one has 10.5 coil). By default, Riese & Müller mount the harder 8.5 coil spring and Pacific Cycles the softer 10.5 coil. There are also several hardness for the rubber damper inside of the front spring but they have little influence on the front suspension, its the spring hardness which is important. But yours seems pretty old and should be replaced.

I have no idea how to mount the SA 3s IGH as mounted on the Pacific Cycles Birdy Touring. In case of change of the rear hub, you should know that Riese & Müller adapt the crank chainline to the rear hub chainline by adding spacer on the bottom bracket, the Birdy Rohloff for instance has different spacers and a different chanline value than the R&M Birdy Touring for 10s cassette.
For me, the best transmission for the Birdy, the one that provide the best efficiency and ride pleasure is the cassette+derailleur type as mounted on the R&M Birdy Touring. I would never mount such an heavy and inefficient SA 3s IGH on a Birdy (nor would I buy a Birdy City with 8s Shimano IGH).