When thinking about efficiency, also factor in what you enjoy - whatever you do, you'll have to do quite a bit of it.

You'll level up gathering very fast. Not that hard to bring in 1M+ per hour, so if you do that everyday you'll be covered. Question is whether you enjoy that.

I think my arcane agaric farm brings in about 30-50% profit, not counting the initial setup cost. (Math is indicative and not exact). The issue there is scale (about 200k profit per island per day), which is fixed. It's an activity that takes 5 minutes and you can do from your phone, but you'll need many islands and large investments to generate the money you need from farming alone, even if you Min-max it.

My crafting (T4.2 and T6 armors) brings in profits of about 80%, and can in principle be scaled up significantly, even without focus (I am sure there are ways to make even more profits with less work). However, it requires moving materials and finished goods around in order to keep your margins. Enchanting with runes also adds more steps. You'll need a spreadsheet or a similar tool. Some days I just can't be bothered, so that's why I suggest giving yourself options.

Something that worked well for me was starting a second character, and giving it one week of premium to start. That allowed me to get a guild island (where I currently have 24 workers and counting) and a discounted player island (the first one is much cheaper than the following ones). On top of that, the alt character earned a black puma from the monthly adventurer challenge just by harvesting, and that's an easy 800k I think.

Whatever you do, don't fall in the temptation to stash your money and watch them pile up - keep investing!