Hi:
We cruised London to Reykjavik (via Ireland and Greenland) in July 2022 on the Voyager; also stopped in Isafordur - loved Iceland. Had only 1 full day (plus departure morning) in Reykjavik; it deserves more! We were on the Splendor for its Maiden voyage and it became our favorite ship (we've done 23 Regent cruises).
In response to your questions, I note that the cruise of interest is "port intensive" with 4 tender ports and only with a long departure day (no included Regent excursions) in Reykjavik - I strongly recommend that you go early or stay over for a day or 2.
We think that the F1 or F2 cabins are the "best" Regent value and choose between them based upon location (I want mid-ship, low in rough seas). They and Concierge have a tub (which we use only as a laundry basket) and a nice shower (but much smaller than that in a Penthouse suite). F1, F2, and Concierge suites are the Cabin size regardless of Deck, but verandahs vary by location.
You have probably been told by now, the RSS Society discount is independent of other discounts ("stackable") and depends only on the cabin category.
Coming from Philadelphia, your Regent Air Business Class flight may be on Icelandic Air - their Business Class seating is more like U.S. domestic First Class than the BC to Europe