Mein Terminplan ist voll, doch ich finde Zeit für einen Kaffee.

When voll follows the verb sein, it functions as a predicative adjective:
• Mein Terminplan ist voll. = “My schedule is full.”
If you place it before the noun, it becomes attributive:
• ein voller Terminplan = “a full/packed schedule.”
Both are correct, but ist voll is the most common way to state that something is full.

In this sentence, doch is a coordinating conjunction meaning “but” or “yet,” introducing a contrast between two main clauses. As a conjunction, it does not trigger inversion, so you keep the normal subject–verb–object order (doch ich finde). You could swap in aber with almost the same meaning:
• Mein Terminplan ist voll, aber ich finde Zeit…

German requires a comma between two independent main clauses when they’re joined by most coordinating conjunctions—doch is one of them. That’s why you write:
• Mein Terminplan ist voll, doch ich finde Zeit…

Yes, but jedoch is more formal and behaves differently in word order. It’s an adverbial conjunction that normally follows a comma and triggers inversion:
• Mein Terminplan ist voll, jedoch finde ich Zeit für einen Kaffee.

• Zeit finden für = “to find time for” (you make or squeeze time despite being busy).
• Zeit haben für = “to have time for” (you already have free time).
So ich finde Zeit implies effort; ich habe Zeit implies availability.

Yes:
• Uncountable (mass noun) when referring to the substance: Ich mag Kaffee.
• Countable when referring to a portion or cup: einen Kaffee (often shorthand for eine Tasse Kaffee).

• Terminplan = an abstract schedule of appointments.
• Terminkalender = a physical or digital calendar for booking appointments.
• Kalender = any calendar (wall, desk, app).
In everyday speech Terminplan and Terminkalender are often interchangeable when talking about your appointments.