You're asking us to second-guess tectonics.js
I have no reason to believe tectonics.js doesn't do a good job, but it appears that it doesn't bother drawing the plate lines for you. That means you're asking us to second-guess what tectonics.js did... assuming it did it correctly and without too many assumptions.
There is a trend in worldbuilding right now to create the most "scientifically plausible world" possible. It's laudable. It's also a bit silly. Worldbuilding is so much more than "scientific reality." In fact, good worldbuilding is knowing when you need to focus on details... and when you shouldn't. Unless your story uses plate tectonics as a specific plot device, you're straining at the proverbial gnat.
Nevertheless, there are some >>basic<< rules that can help you
Always start with a chart of Earth's plates.
Plates always have low spots (oceans) and high spots (land.)
Plates move and rotate. When they move away from one another, they create chasms (e.g., the Mariana Trench). When they collide they create mountains (e.g., the Himalayan Mountains).
Volcanoes frequently happen along plate boundaries (e.g., islands and mountains). This is especially true when the plate is rotating rather than moving.
Plates rarely cross landmasses. It isn't that they can't, it's that water erosion usually eats away at the sides of the land masses, leaving seas between the two land masses with a plate division along the sea. Thus, a plate bisecting a continent (incredibly rare) would have been two plates pushing together for a very long time, resulting in a substantial mountain range bisecting the continent.
Though hard to imagine, it is believed the U.S. Rocky Mountains (forming the U.S. Continental Divide) were caused by the shockwaves of two plates colliding. In other words, mountains don't form only because of two plates pushing against one another. Mountains (or, generically, "high spots") also form due to meteor strikes or, perceptually, from water erosion (e.g., draining Lake Bonneville) or wind erosion (U.S. southwestern bluffs) due to low-density rock being blown away from high-density rock. You can also have sink holes due to the type of rock (e.g., limestone). In short, you need to realize that you'll have high spots (mountains) and low spots (lakes...) in locations other than plate boundaries.
Using these rules, you can start with some basic assumptions:
a. Your average plate will have ocean encompassing a landmass.
b. In the majority of cases, touching landmasses will touch lightly or with small stretches of land (e.g., an isthmus). Note that an isthmus can form due to soil buildup around the two plate boundaries having little or nothing to do with the plate boundaries. I.E., ocean currents deposit sediment that reinforces the currents' behavior until an isthmus forms.
Your map is actually very hard to work with, and it's the fault of the assumptions made by tectonics.js
Where are the polar caps? Polar caps help identify how much water is available. In a warm year there's less cap and more water, meaning greater separation between land masses. In a cold year there's more cap, less water, and more connectivity between the land masses.
The outline map really is only showing us coast lines. It doesn't show us topography, which is really what we need to identify plates.
How much water is on the planet? If we knew the topography, then the amount of water would dictate land mass connectivity save for where two plates have been colliding for a very long time.
In the end, sketch some quick boundaries based on the map of Earth's plates and the rules I've given you... and move on.
As I said, unless tectonic geology is a critical plot point or plot device in your story, you're straining at a highly technical bit of science that humans don't yet completely understand in the first place, all for (what I suspect is) a goal of dubious value. If your goal is a map, it would be easier to sketch in mountains, lakes, cliffs, and rivers where you need them based on narrative necessity and move on.
