Opening as a Beginner: Mainlines or Offbeaten path? I am approximately 1500 OTB (Lichess Classical 1750, chess.com 1540, internal rating in my club ~1500), and of course I would like to improve. Work and family are demanding, so naturally I would like to make the most of my limited study time.

I do not think that I gain a lot by spending time on studying openings. On the other hand, I want to and like to play tactical lines in order to improve my tactical vision.

Currently I play 1.e4 as white (Evans Gambit, Fried Liver, Open Sicilian, Korchnoi Gambit, improvise on everything else), and as black I play the Qd8 Scandinavian against 1e4 and QGA against 1d4.

I wonder whether I should switch to more tactical lines as black, for example the Portugese Gambit against 1e4 as presented by Smerdon ("Smerdon's Scandinavian) or recently Vincent Moret in "My first chess opening repertoire for black". Moret also suggests the Albin Countergambit against 1d4.

I do feel however, that these openings are far from trivial. For example in the Evans Gambit, a lot of good moves come quite naturally, whereas in the Portugese Gambit there are a lot of nuances and move orders to remember (should I put the queen on f5 or on h5?) etc., and if you make one bad move you are very quickly on the losing side. And there are lots of chances to make bad moves.

So in summary: To improve my tactical vision and my game in general, should an amateur like myself invest time in studying these sharp and aggressive offbeat opening lines, or rather play more solid lines and spend the time on studying the endgame and middlegame?

I'd be especially interested in stronger players experience, i.e. how they improved.
In general, independently of playstyle, I'd advise every beginner who seriously wants to improve in the long run to favor "popular" openings over "offbeat" ones.

Offbeat openings are (usually) considered offbeat for a reason. That is, usually there is some way for the opponent to "defuse" them. Of course, that doesn't mean they are unplayable, and at a ~1500 level, most opponents won't know about that way anyways. They will at most have learned some variations of the main openings and have no real clue about playing against the Evans, Albin, etc. gambits. If you are properly prepared, you can exploit that, and these offbeat openings will often net you more wins compared to the popular ones in the beginning. However, in the long run, you will run into more and more opponents who DO know them, and some years down the line you may face the dilemma that these openings slow down your progress. 

Popular openings on the other hand will usually be less impressive in the beginning, point wise. However, if you find one that you like, you will essentially have found a buddy for the rest of your life. Similar to the offbeat openings, they are (usually) popular for good reason. Having already lived under the microscope of thousands of master for decades, it's unlikely that they will suddenly be "refuted" (that risk is much higher in offbeat openings). Yes, the amount of theory may be frighteningly big, but so is the potential reward. After all, while many different lines mean that you have to have much more to learn, they also mean that you have much more to learn: You get a much broader scope of middle game positions you can potentially reach, and learning how to handle them is very beneficial for your overall chess skill. You don't have to know all little lines from the beginning (your opponents won't either), just start small and build up the tree move by move over time. Honed over the years, a popular opening is guaranteed to at some point surpass the results of offbeat openings.

My personal approach is to play popular openings, but slightly offbeat lines within these openings. That way, I still retain some surprise factor (and decrease the risk that an opponent "out-prepares" me), while still having the safety to play a very sound opening that I can fall back on. It's much easier to just switch to a different line at move 5 (that most likely uses at least some of the motives that I already know from my previous choice of line - not to mention that all side lines before move 5 stay the same) than to switch my entire opening at move 1 (that I will for the most part be completely unfamiliar with). 

In your case, I agree with the other answers. Sicilian (Najdorf/Sveshnikov), Slav and Caro-Kann are all fine choices. I mean, you can play stuff like the Portuguese gambit every now and then to have a little fun, that certainly doesn't hurt ... but I wouldn't devote a lot of time into it.