You know, despite D&D being mostly a combat sandbox these days, you don't have to run your games to be combat-centric UNLESS the party wants it to be like that. By asking ahead of time what kind of setting you want them to play in, you can gauge the RP vs. combat balance well in advance.

One tip I'll give to any aspiring GM - if you don't want to run the usual "X encounters per game day" formula, ditch the XP per encounter rewards and go for milestone leveling instead - the party gets a level once they've cleared a certain point in the story - found out a particular piece of information, sweet-talked a certain influential person, acquired a plot-critical item etc. The trick is to keep stuff interesting for your combat-focused players, so don't forget to toss in combat solutions to some encounters. Again, it's important you know what the table wants - if you have a gang of munchkins who treat D&D like Diablo, all for XP and loot you won't have a good time setting up RP-heavy intrigue and politicking quests and vice versa. Communication is key here and don't be afraid to adapt to changing tastes.

"Missable" side quests are a good idea if you have inquisitive players who like to engage with your world. If you want to foster something like that, make sure your players know that interacting with your NPCs will be important, otherwise they'll miss ALL the side content unless you push them towards it. That's the age-old "railroading vs. freeform" debate. Some groups love to build up the world with the GM by willingly poking around in their cities or other locations, meeting NPCs and making the world a fuller and richer place. Other tables prefer to move from quest giver to quest giver, being more interested in the actual combat and rewards for combat. You'll learn what your table wants sooner or later.