Your SNES Jr needs an RGB mod if you want to use it with the OSSC. Do you know which revision your original SNES is?
The best RGB SCART cables are from Retro-Access. They make full shielded coax cables and also allow for easy access to the inside of the console connector if you ever need it.
Cheap SCART switches usually cause some picture quality loss. There is a high quality SCART switch being sold on shmups but it is not cheap. https://shmups.system11.org/viewtopic.php?f=6&t=50851
Don’t forget that if you use a SCART switch then you also need a shielded cable from the switch to the OSSC: https://retro-access.com/collections/scart-to-scart-jp-21-to-scart-hookups-etc/products/copy-of-jp-21-to-scart-cable-pro-coaxial-multicore-for-switch-boxes