I think Duolingo is good for getting you past complete beginner but to go deeper I recommend Anki.
2. It's not bad to forget words during the learning process.
3. Purely personal: SRS reviewing is the ultimate bore, however you design your cards. And making the cards is even more of a bore. It's the opposite of fun.
My background is that I’ve used it while studying Korean. I’m at about 2,000 korean words now, of which Anki has helped me a lot. Each day I review 50 words and add 20 new ones. It sucks but it works.
As you see new words in reading material, etc, you can add it to Anki. That’s supposed to be how it’s done, but there are premade decks also.
The real beauty lies in anki’s customization. You can add pictures or custom fields and style them. You can reverse the deck so you’re seeing one side or the other when reviewing.
Anki is NOT helpful with a word’s context if you don’t know it or don’t add sample sentences. Which is why people recommend building your own deck over time by reading short stories or news, etc, and adding every word you don’t know, rather than using a premade 5k word deck.
If you have any other questions feel free to ask.