As I mentioned on Facebook this morning, I tend to assume that the ship’s Commanding Officer chooses how their ship is staffed, and who is amongst the senior staff. In-game, I tend to leave that thing up to the group and whatever roles the group themselves picks (and how they construct their bridge).
Personally speaking, I don’t regard “Tactical Officer” as a full-time distinct role in the same way as “Chief of Security” is - the Chief of Security is always in charge of the ship’s Security department, but the Tactical Officer is only really the Tactical Officer when they’re stood at the Tactical station on the bridge. In both TNG and Voyager, the vessel’s primary Tactical Officer (the person normally at Tactical) is the Chief of Security, who is relieved by one of the relief bridge officers when they step away (same as with the Conn or Ops), and their Tactical stations allow them access to various Security oversight functions as well as communications systems (because a starship sends and receives secure or encrypted communications… and Worf’s normally the one answering or sending hails in TNG). Worf also seems to be responsible for training and supervising junior bridge officers.
TOS didn’t have a dedicated Tactical officer either - weapons were handled from either helm or navigation - and while Enterprise is the closest to having a ‘pure’ Tactical officer, Reed’s role is Armoury Officer, so operating the ship’s weapons is an extension of his responsibility for all weapons aboard the ship… and even then, he still oversees ship security until the MACOs arrive.
When we get to DS9 (the weird one), the station doesn’t have a dedicated tactical officer at all - one of Kira, O’Brien, or Dax operates the station’s weapons, with Odo’s security/law enforcement side emphasised in part because of the civilian and mercantile nature of the station. On the Defiant, there are two tactical stations, with both Worf (Strategic Operations Officer) and Kira (Station XO) commonly staffing one or both at various points… even on a tiny ship with an overabundance of weapons, there’s no dedicated Tactical Officer (though the Defiant’s crew is a bit ad-hoc, with characters taking on whatever jobs they need to at any given moment).
Overall, though, I assume that Starfleet officers, especially Main Character ones, are absurdly omnicompetent, able to rise to the occasion and do the jobs demanded of them, so lots of officers have more than one job aboard their ship. It’s a universe where ten year old children complain about having to learn Calculus, building a subspace radio is an unremarkable thing for a teenager to do (as mentioned by O’Brien), and individual medical officers are skilled in treating dozens of different species.