I don't have that style of portlight, rather mine has the exterior trim ring.
I have removed mine a couple of times.
Cutting and prying caulk can be tough.
I use the Stanley wonderbar 55-045 . I now have 6 of them but the first three were the best. Chromed, [don't really care about the chrome] and the big pry end has a rolled and thinned heel rather than just a bend which many competitors do. That rolled heel makes a big difference in the effectiveness of the bar. I then ground down the tip thinning it out to a hair less than 1/16" on two of them so they can be worked behind the rings more easily.
Then use it to pry open a gap so a sharp cutting knife can be used to cut the caulk. Then I used the third unthinned one to open and hold the gap so I could move along and open the gap and cut again. It was slow but it worked. I also used pieces of aluminum and many times one of the bars to protect the surface I was prying against otherwise the bars can leave crush marks in wood.
To do the caulk cutting I use use the big Olfa knives, 3/4" and 1" blades.
Need lots of blades.
Be carefull as they are sharp and the constant bending will eventually break them.
There are two versions of the bars, the better chromed ones and the not quite so well done black and yellow ones.
I also have 3 of the black/yellow ones but the tips are much thicker as Stanley has decided to not do any thinning on these. TYhey still have the rolled heel. So i'm doing my own thinning and they work fine too, just more effort to get them to do so.
I have also used my oscillating tool, ROckwell, with a toothed blade which did the job but did some damage to both the trim ring and the fiberglass so the next time I will use the scraper blade, no teeth, along with the bars.
Next time I will also tape some of the thin kitchen cutting sheets to the surface the Rockwell rests against as the tool left some marks. THis stuff is tough and can be scissor cut.
I was poking today and see Amazon offers the black/yellow bars.
The older chromed ones are still around on Ebay.
I no longer use caulk for stuff like this but rather the butyl rubber tape from CMS on his site. It's now Marine How To
Do yourself a favour and spend some time reading his site as he offers a huge amount of good info.