Since you're talking about portals, I guess you're wanting to make an overworld farm? Those are actually quite inefficient unless you really, really optimize the design and placement of portals, meaning, you maximize the number of portal blocks you can fit inside of a 128 block radius. ilmango's design used around 25,000 obsidian, which he estimated as 12-13 hours of mining with Eff V and Haste II beacon. That is a lot of obsidian, which doesn't meet your other criteria of being [not] mega hard to build.
For a much more reasonable design, ilmango recently published an excellent nether roof gold farm design for 1.12 that is very easy to build (compared to other designs) and gives close to max XP and gold drops. (He then published the "expert" version that does max out XP, but it is significantly more difficult to build.) I would recommend the non-expert version for what you want. If you have any experience following Minecraft build tutorials, you'll be raking in the gold in a few hours at most.
Have a look at that, and if it is too difficult to build, I don't have a good answer for you. You could just keep building overworld portals and slowly build it up. I wouldn't recommend it, but if you want to try it out, I'd suggest you have a look at where he builds his huge overworld farm. Even if you go smaller, he discusses many of the key mechanics and pitfalls he discovered when designing it.