Not sure about the quartz, but I ran into the same issues with laminate pricing re: your island. Laminate is easier in "regular" depths (ie: 25.5"), so to have a 6x6 island will require some more custom work. Also, the edge goes around the whole island, so essentially you have ~24 feet of edge as compared to 12 feet on your perimeter. Probably that's what's happening with the quartz as well (the additional edging) except that is does come in wider widths.

I have replaced laminate countertops, and you are right that it isn't a very big project. There are a few things to think about though. We were replacing the backsplash as well so this wasn't a big deal to us, but it probably isn't possible to save the backsplash when replacing the countertop. Unless you can do it yourself there is an additional cost of removing and replacing the sink. You also want to carefully consider if you want to throw out over $2000 in laminate down the road.

What is the quartz edge on your island? It is possible that you could save a bundle with a simpler edge. Also, shop around for different quartz installers and different quartz brands. There is sometimes a big difference in cost.

You are probably seeing a higher than expect cost for the island and bathrooms because of the cost of the sink cutouts.

For bathrooms we have had good luck with "cultured marble" vanity tops with integrated sinks. If you get white in a matte finish it doesn't look cheesy (to me) and they are very low maintenance.

Good luck!

Laminate width maxes out at 60" so your 72" island is suffering from both a seam and a custom labor charge because of the odd shape. I would actually have expected the island to be higher priced given those factors. Is there no way to reduce it's size? As described, it will be extremely difficult to clean and awkward to use.

Also, you are choosing HD laminate which is the highest priced laminate. By the time you add on an upgraded edge, most laminate quotes can be almost as expensive as the low cost granites. If you want to keep costs down, choose a "Group A" laminate (or look at a inexpensive granite) and pick a plain waterfall postformed edge. The island will be a problem no matter what type of counter you choose, so do look at redesigning it. If that cannot be done, then look at doing an alternative material on it like butcherblock or solid surface (more suited to odd shapes and inconspicuous seams) , and go ahead and do it in that material now. That will help you to cut your budget, but not feel as though you've wasted it on the island.