Your plan sounds in equal measures fun, challenging, uncomfortable and impossible.

"Traveling overland to Mauaus" means by motorbike? You're buying or bringing your own? "Sightseeing in the Roraimas" means crossing into Venezuela? You understand the current political and economic issues in that country? Or staying in Roraima Province in Brazil? "Slow and relaxed" means what, exactly? Have you experience in overland travel by bike? Do you enjoy it? And what do you want to spend your time actually doing: museums (which are mostly small and few in number along your route, but they do exist), trekking, shopping, local culture, fishing, satellite launch-viewing....?

The obvious difficulty with your plans is this concept of renting a "dinghy," which if you pursued it would transform the whole journey into a world-class epic adventure. That's a thousand km, more or less, along the Amazon, and although there are no waterfalls neither is there much infrastructure available to the casual boater. I'm guessing you'll have enough on your plate if you substitute a river boat for the dinghy portion; as an added benefit, your wife will still think you're crazy.

FWIW, my rough estimate suggests this is a three-week trip provided you keep moving along--longer (and potentially much longer) if you like to dally.

There is easily enough to do within several days' riding from Paramaribo, depending on your interests--four countries, a range of activities, cultures, historical remnants, natural wonders. Notably lacking--good motorcycling roads, mountains, and aside from one or two roads per country, infrastructure. It's not difficult to learn more about what's available, on this site or via Google--just settle in and start searching for it.

Hope that's helpful.

Mark