AFAICU SwOS is just fine (and simpler) if the switch is used as “simple” managed switch, but with RoS you have more possibilities, if needed.
The way a router (designed as a router) and a switch (designed as a switch) differ is in the way the ports are connected to the CPU and between them (dedicated switch chip), a switch is faster at switching, and router is faster at routing (though often a same device is used in both roles, and there are some “compromise” devices such as the CRS series, which are mainly switch but with some routing capabilities and the L0009 that is advertised as router, but actually it works better as a switch), it depends.
Two SFP’s should (could) be useful to have one connection “upstream” (towards router/internet/outside) and a second one “downstream” (a second switch in the same or another room) or (say) two connections upstream (2 ISP routers in failover or load balancing).
Fiber is in its own a whole world, a very deep rabbit hole, it depends a lot on your physical setup, you cannot (yet) have DIY “fiber patches” (the tools/splicer/taper/etc. are simply too expensive, and even if it not rocket science you need to learn using them), so you need pre-made ones (that you cannot normally pass through existing pipes/conduits as the terminations are relatively bulky and fiber is (relatively) fragile, if you need that, have a specialized technician/electrician do the terminations.
Then there is the issue with SFP’s, finding good ones that actually are compatible with Mikrotik devices (unless you use Mikrotik SFP’s) may be a challenge in itself.
Same goes for DAC cables, Direct Attachment Copper, in practice a copper patch with SFP’s already attached at the two ends, good enough to connect two high speed devices for a short distance, and creating no or very little heat, so as an example suitable in your case if you have the main router and the main switch in the same room or however at a distance below 10 m or so and can pass the cable with the very bulky SFP attached.
About fiber, the basic distinction is between single mode and multimode, but there are a number of other characteristics when choosing a fiber patch cable, both single mode and multimode will do for the distances you likely have in your home (meters, not kilometers), multimode is also a bit cheaper/more common (please read as more likely to be reused in other projects):
https://community.fs.com/article/singlemode-sfp-vs-multimode-sfp-what-is-the-difference.html
You will need to do some research and learning before ordering/buying stuff.
I may be wrong, but I think there is only the CRS310:
https://mikrotik.com/product/crs310_8g_2s_in
with 2.5 Gb ports, maybe you can rethink your topology and do everything with two of them (while more ports are always better a fastish 8 port 2.5 GB switch may be a better investment, as it is a more flexible/reusable in other projects device).
Or you could get a (cheaper) unmanaged 2.5Gb 5 or 8 port switch from other vendors (Tp-Link or Zyxel are common choices), if all you need is a dumb switch between a bunch of devices.
Your network upgrade ain’t gonna be cheap, both in money and time needed to study the matter.