I need a server for OpenCart, with approximately 30,000–40,000 products and several thousand visitors per day.

I'm looking at 4–6 CPUs / 8–16 GB RAM / 100–150 GB NVMe.

I've seen some interesting options on hostpro.com, but I can't decide between 8 and 16 GB of RAM.

Anyone with experience running similar stores—what configuration would you choose right now?

I'd draw your attention to VPS accounts which justvps.pro and 4goodhosting.com provide.
They are the best so far and I believe they are the best. Excellent customer support, reliable servers, fair prices, and all the features you will ever need.