I haven't really thought about which faction to join yet. Naturally, I don't really like either of them and would prefer to work for all or none.
Well, Esteban doesn't care about forcing you into his ranks because he already has a lot of people - former city guards and the like - and the only things he cares about are his gold and the city. On the other hand, Mendoza desperately needs manpower to look for the items he needs and to stop Esteban's frequent raids and intrigues. You'll eventually find out that both factions/leaders are just ruthless bastards and they mostly care about themselves, even if they have some "common good" motives.
But going for strength and swords just seemed more useful out there in the wilderness and on my own (seeing that magic is hard to come by and long range combat doesn't help you a lot when enemies always come running at you in packs).
This is when character planning is important. You have to plan way forward. I usually only learn STR to ~30 and minor skills (lockpicking, pickpocketing, smithing, prospecting...) before joining a faction. Maybe a few levels of swordfighting (especially in my bandit playthroughs). That's just about enough to clear most of the island (except for ashbeasts, ogres etc.) if your (player's, not character's) skill in fighting is at least average. This way you can learn 6-10 levels of crystal magic instantly once you join the Order. And believe me, Magic Bullet lvl10 and a bit of mana is probably the easiest way to finish the game.
And seeing that the order always tries to force me into their ranks, while the bandits don't really care, I guess that yes, I'm more likely to side with them, even though I hate missing out on interesting monastery and magic quests, as I'm unlikely to replay the whole game
Your biggest mistake so far. :P
But really now. I'd honestly recommend you at least one more playthrough, as a mage. There isn't really that much of a difference between a mage and a soldier of the Inquisition (mages offer maybe two or three more quests and, of course, a slightly different "career"), at least not to the extent of another playthrough if you don't want to finish even two.
Plus, mages are generally wiser and kinder than the Inquisition and definitely won't try to "force" you into their ranks - in fact, they have no reason to since the only way to join them is to willingly
assist the Order in Harbor City, and that suggests you want
to become one of them. ;)
I think I've finished all the bandit and farm quests except for those that would require me to visit the enter the city or the monastery (seek out treasure hunters for Finch, talk to the mage in the monastery). Not sure what other quests you get from Finch, but I already conquered a temple for him.
That's fine. The problem is that you have to finish the first quest before joining a faction, since once you're the Don's emissary or a member of the Order, you can't go around killing your colleagues (or people you negotiate with) and stealing their treasures. :P
The only quests that are still relating to the wilderness now are (a) "investigate a mysterious temple in the east for Finch", but that's the same temple he wants those treasure hunters for, so I'm not sure if I'm supposed to get them first from the city or investigate on my own and call them later...
You'll have to go there before you can hire them, because they won't leave unless they know what they're facing. :P
(b) Marvin's quest about the rings of the undead, I've already killed a few undead lords and found 3 or 4 of the six rings and I've located Patroscon, but after defeating him, I reloaded an earlier save game since it seemed that I would be skipping some quest stages by killing him this early. I read that the whole quest starts with Marvin's mother in the city, and I didn't get a quest to kill Patroscon yet either, just to collect the rings. So that's what made me think I'd be missing out on some bits of story and a few xp if I'd just do stuff like that all on my own without getting any quests for it.
Eh, that doesn't really make a difference. Reward is the same and the only thing you'll miss is a single Leon's/Marvin's (different names in different language versions) line about how he still has nightmares after collecting all six rings. Also, his mother only wants to know he's safe, she doesn't really care about anything else.
Assuming I'd never replay the game, would you say one faction offers more interesting content and more of it than another (e.g. if I'd join the order, would I get all the monastery stuff on top
of what I'd get as a bandit, or would I get access to equally cool and exclusive stuff as a bandit as well)?
There are slight differences in the plot, but neither faction is really more interesting or cool. Joining a faction always disables some teachers, quests, armors, (possibly) items etc. from the other one(s). Overally, mages offer the most stuff, skills and quests, but not very
significantly more (well, except for the magic, obviously, but even the Inquisiton can use crystal magic).
And I gather from your usual playthrough that it does
make sense to postpone the joining of a faction as long as I'm still able to do other stuff on the island? That there's a high risk of missing out on a good chunk of island exploration in joining them too early? And that it's worth paying the extra 100 gold for re-entering the city after escaping it, in order to get the most out of the game?
No, no. That's just my completionist style. You can always explore everything (except for main-quest-blocked areas), even after joining any faction.
As a sidenote, if all of that is true, it seems like a quite weird design to me, free exploration and forced story / C&C competing with each other, with the game seemingly pushing you into directions that will make you miss out on a lot if you follow them early. And also the thing about officially being forced to stay within that city for such a long time when it's just such a small part of the whole island ...
But it offers more quests than any other location in the game - and you can get every single one of them in the first chapter.
Also, Piranha Bytes were always like this. In G1, you could join the Old Camp before visiting any other location in the game; in G2, you could join the militia less than an hour into the game - but it would prevent you from completing a considerable number of other quests. Gothics (and Risen) were always like "Do anything you want, go anywhere you want, join anybody you want, all of it whenever you want - but be prepared to live with your decisions."