What background to play?

Depends really on what kind of story you wish to play

From a general perspective, playing a human is the most common to play as, mostly because the story was initially designed to play only as a human (like DA2)
However, given the recent revelations, playing an elf is also very interesting since you get to know a lot about their background (Inquisition only has a dalish background)
However, imo. Qunari have the most fun in terms of war-table missions, they get some really fun dialogue as well.

It also depends on who you wish to romance, since a few of them are race-gated (them being Solas and Cullen, Solas is race-gated for female elves only, and Cullen for female elves and humans only)

I've so far finished a game with both an elven mage and a qunari mage, both were a lot of fun to play, especially the latter.
Seeing as you already have an elf in your world state for origins, and DA2 is obligated human, I'd recommend playing either a dwarf or qunari.

I've finished with F Human Mage, F Elven Mage, M Elven Mage, F. Human Rogue. Have partially played Two Hander Warrior Human Male. I enjoyed my first run as a F. Human mage, and really extra enjoyed my F. Human Rogue.

I think if you are heavy into RP you should probably play a Dalish elf, either gender, because this is a very Dalish Lore heavy game. Especially one of the DLCs, I was actually disappointed I'd seen that lore as a human Inky instead of as an elf. I don't think you have to be a mage, and I preferred Tempest Rogue followed by two handed warrior as the most entertaining classes. I didn't enjoy any version of mage, I always felt two far away from the action, and defaulted to being a shield caster between some lame attacks.

I think being a dwarf or a Kosith (the actual species name. Qunari is like Christian, and you don't get to follow the Qun) is very under utilized regarding anyone commenting on it. Additioanlly, the cinematography for a dwarf is effed up from literally the first scene. Most people say a dwarf Inky is like playing a short human, and the latter as a very Tall Human.

Re. Romances (assuming you are going with a dalish)
If you choose female Dalish, I don't recommend the Solas romance, it literally has the fewest scenes and modifications. You will literally feel like you romanced no one, IMO, although there are a lot of devoted Solas fans who fill in all the gaps with their imagination. As a F. Dalish, I don't recommend Sera either as a romance. As a M. Dalish, Cassandra is not blatantly horrible, but there are a few weird comments here and there.

Any significant difference in roleplaying between a Qunari Mage or other classes?

From a gameplay perspective I'm reluctant because I have heard through osmosis you can't hit people with your staff in melee combat and Arcane Warrior's not an option once again.

I have also heard rather bizarrely that if playing Warrior or Rogue you have to pick a style of combat and have no options to deviate after that point for example either longsword and shield or just greatsword...

Being a horned being will get like, maybe 5 comments, usually from specific companions. Being a mage will come up often, and you even get extra dialogue options for being one.

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We're both right, I guess. The Qun religion doesn't give much attention to race? so they never bothered to officially denote their primary species by a particular name. you are just born into the Qun or join the Qun at a later age,. I still prefer calling them kossith until corrected by in game cannon, perhaps in DA4 when we start running into a lot of them. Doesn't Iron Bull straight up reject an Q Inky? by saying something like, "you're not one of us." but they are the same species

I have heard that in a later DLC you get Nero'd and taken out of the fight permanently.

But surely that does not really make a lot of sense for a Mage.

But for a Warrior or Rogue it does.

Especially if they're a two hander?

I'm thinking big Qunari sword lady, might fit the stoicism and blandness of the Inquisitor's personality, and be an interesting contrast with being an agent of the Chantry.

Thank you for the advice everyone.

No I'd say it's moer jus tth etemplars really rather than everyone. Most of the mages don't really want to fight that's why they hid in Redcliffe to try to sta yout of everybody's way. It's more the mages don't like how they've been treated lately a tleas t up t oDAI an dthe onl ywa ythe yewre going t obrea kfree was t oembrace their powers asnd break awa yfrom their accor5d with the templars. I like playing mages most out of the 3 main classes. I think mos trecognise that no tall mages aer bad some may go to ofar yes like wha tOrsino and Uldred did but not al mages aer bad. Both Leliana and Cassandra recognise this and I think in some cases so does Cullen even if he does show an element of resentment toward them given what happened to him in DAO. Mages as whole in these games are quiet good fun to play. In DAI I'd say it kind of fits wel because of how the elven ruins and all that fit in too. Was thinking of playing a mqage on m ynext playthrough of DAI.  I'm not playing one atm as I'm playing an elven warrior right now but I do enjoy playing mages the most.

A lot of people will say a female elf mage who romances Solas, but I really recommend just choosing whatever appeals to you the most.