Calendar/Weather VS worldbuilding

There's one called Simple Worldbuilding System which is a bare bones system, you can make a character sheet there and set is as a template, add rolling macros and just play with that.

Allow me to direct you to the Simple Worldbuilding System which was designed for EXACTLY that purpose.

So I'm using the Simple Worldbuilding System, since I'm homebrewing my own campaign entirely. For the most part, I've been fine with everything the system has been able to do for me. There's really only one category that's been lacking for me: items.

Foundry is great if there's a module and probably not worth the effort if there isn't. You can try putting together what you need from either https://gitlab.com/rolnl/sandbox-system-builder/-/tree/master or https://github.com/foundryvtt/worldbuilding both if which have some youtube tutorials.

It’s a VTT but maybe Foundry using the Simple Worldbuilding System? That is the generic system for Foundry not tied to a specific game.

Also, you could try using the Simple World building module to get a start making your perfect system. https://foundryvtt.com/packages/worldbuilding

You can do as little or as much as you want. Tokens are straightforward uploads/choices. My suggestion, with as much custom material as you have, would be to use Simple Worldbuilding System and PDF Character Sheet. That will allow you to start off with something manageable while you learn how to add classes, feats, and races to the 5e system. Custom mechanics will require programming or using something like Sandbox. I’m pretty happy doing theatre of the mind and using Foundry for combat tracking and mapping.

For Gold & Glory is an implementation of a system heavily based on AD&D 2e and can function as a good core for it. Simple and other system agnostic options can also be used for it. If you're wanting full in-built integration and automation though, your only option is, unfortunately, FGU still. Only VTT place its ever actually been licensed out to.

This is sort of true and sort of not. There is a Simple Worldbuilding System module that basically gives you a "blank slate," similar to what you have in Roll20 if you don't select a system.