Raymondk1_,
I don't have an R series mirrorless camera and therefore will not offer an opinion and certainly not a recommendation on an R series lens. I would like to respond to your question in general terms based on my experience and I hope you find it helpful.
For portraits, consider a lens that has a realtively wide aperture - say 2.8 or wider. You can Google portrait photograpy for helpful advice on shooting portraits and portrait technique and also watch Youtube videos on the subject. In my opinion, one of the common recommendation is that it's a good idea to use a wide aperture lens to isolate the subject from the background and render faces and facial features in a more pleasing manner. For an APS-C (crop sensor) camera like the R50, my opinion is that a focal length from approx. 24mm to 85mm would work for you but as they say, "your mileage may vary". I don't think it matters a great deal if the lens is a zoom lens or a fixed focal length lens, although a wide aperture zoom lens costs quite a bit more than a wide aperture fixed focal length lens within this range. Fixed focal lenth lenses in that range are usually are pretty reasonable but a more expensive zoom may be more versatile.
I don't shoot a lot of architecture, but my thought is that wider angle lens with a focal length of 24mm or less might work. That's where the versatility of a zoom lens may justify the greater cost of the zoom because that zoom may be able to cover you both for portraits and achitecture. As I said, I don't use an mirrorless camera but Sigma recently introduced an 18-50mm f2.8 lens for Canon RF-S bodies. The opinions and previews I saw on the Youtube for that lens were favorable and you may want to check them out if the Sigma lens sounds interesting to you.
If you think that a wider angle zoom may be more useful for architecture and depending how close or far away you can get from the building or structure you wish to photograph, there are some really nice zoom lenses available that cover 10-18mm range. Those may be good for both interior and exterior architectural shots and may be worth considering.
Hope the above is useful.
Good luck!
LZ
