I don't think that anyone, here, (not any of the regulars, anyway) would argue with you over the authenticity point.
I wouldn't entertain wanting to learn AT from 90%+ studios. the very fact that they sell packages of lessons at all would tell me all I needed to make that decision. If by showiness, you mean using elements of Fantasia or stage tango to make the 'moves' more exciting (for which, read saleable), then that's about what I would expect, but of course, that style of dance has really nothing at all to do with social tango, where even boleos and ganchos may not be appropriate for much of the time (depending upon where you dance).
Take a step back, and review your goals. If you want to be able to dance with the members of your own local tango community (and not tango at your studio parties), then you need to find out where they dance. Go along, and find out about the local scene. Ask the organiser of the event what tuition options there are. Most people are pretty helpful in this way, and want to grow their local communities. You may find that there is a whole network of dancers that operates completely outside the dance studio model, and it may be a world of which you have previously been quite unaware.
Joining it may be hard work. People can be very intimidating, and I think tango is not very welcoming to beginners. There seem to be a good number of initiation rites which have to be faced and worked through before anyone will take you seriously. BR dancers have a hard time: possibly this is because they tend to think that they know it all already, and just have to adapt a bit (wrong!), more likely because they have not really acquired much proper technique and they've learned the BR equivalent of the 8CB way of dancing tango (ie they can sort of stumble around the floor, but with no understanding of how things work, and no proper foundation of technique). They adopt a sort of pastiche BR posture (rightly, an object of derision for most ATers), but know no better. It is a real obstacle to making early progress, not least because the tango dancers don't know anything about BR technique, either, and the things about which they express opinions very freely are things about which they know next-to-nothing. Attempting the crossover is hard: you're on your own.
Tango is very pattern-based, but not in quite the way you might expect, if you come from the world of the standardised dances. There is a whole range of opinion about what is good and what is not, and it is very confusing to the beginner to know where to begin in terms of whose advice you might trust enough to invest your own time and effort in following. Your best guide will be the scene local to you. Generally, I would expect you to find that the tango community will have come together around a fairly narrow band in the broad spectrum of styles, and what is danced by the locals is what you will have to dance, if you want to dance tango in the company of others.
Later on, you can (and should) find your own style, which may be more of the same, but may be different. If the latter, there are other communities around that will be just as ready to intimidate you as you seek to join them. It's all part of the experience.
