I would argue that making it too detailed will cause more problems than it solves. It encourages you to focus and trust the estimates more however the model is still far from reality. People may quit, people may get tired of working on something, they may realize they don't like working on something, some employees might be experts informally supporting other projects, some projects may not be as good for resumes or promotions, onboarding time is different for different people, some people really don't work well split across projects, etc.
The goal there is not to provide a precise schedule, but define budget, agree (plan) to what end the time of people in the team (capacity) will be used. By connecting to such systems, you might gain accuracy, but you won't gain useful precision.
I've seen resource-management[1] (formerly 10000ft) used in the past and it seems to do the job, but I wasn't an active user so I don't know all the ins and outs.
You want a database that is connected to approval processes, auditing, pay stubs, people's calendars, etc.