I'm looking at moving our on-premise to cloud but haven't yet, but we have about 5,000 active users a year with peaks of 80 concurrent or so and use the following configuration without any problems. You could consider this an upper limit on what you may need?
Web server: 10 vcpu, 32 GB ram
The disk usage will depend on the files you are loading in. Our Moodle site uses about 72 GB of storage after close to 7 years of use, but we typically don't upload videos and deliver those through Vimeo/Youtube.
For DB, I would just use their managed DB server offering and do something like 4-8 vcores on the general purpose or memory optimized tiers.