Hello Franny and Welcome
SD is a very different experience but one that DW and I really value.
There are many shaded areas at or near the pool and the staff will bring out an umbrella for you. That being said the pool is not always the center of the experience. There are shaded areas topside near the bar, at the bow, and many other locations. You will always be able to find a comfortable area to relax, read, chat etc.
The internet is fine and they have recently upgraded to Starlink. I believe the plan is per cabin, not per device. Also in the Caribbean you are almost always in range of cell service so you will have LTE as well (make sure to enable proper plan before you leave).
There is definitely more motion but it is at a higher rotational frequency than on the bigger ships (the larger ships tend to "wallow"). The only area I have ever found it a problem is entering or leaving PR. Everyone is different of course but you should be alright. The ship is not tall enough that deck matters, what does matter is how close you are to amidship. Closer to the middle is best, avoid the bow or stern if concerned.
Most gentlemen where a sportscoat in the evening at least for cocktails, later it is fine to wear "country club casual"
It is very much a line where you need to "make your own fun". They will certainly suggest activities and help arrange them but there is very little in the way of organized activities. No trivia, etc. There is a cocktail hour in the evening. What you will find is that everyone is friendly and engaging.
Hope we will see you!