Maybe im slow. Ive been using this app for a month or so and ive still not finished learning all the beginner intro words. It took me a while to get katakana and hiragana and i deliberately only learn 4 new words a day as i am not good at remembering vocab.
I have still managed to do several grammar lessons and progress and understand how to use certain particals and some conjugations. Obviously i am quite reliant on trial and error AND the translated hints the grammar questions provide.
is this a waste of time or bad learning? Should i focus my efforts purely on vocab?
Grammar and vocabulary come in pair, the both are needed, in any language.
if you know a lot of words but no grammar, you can't communicate. Even with very basic thoughts. If you just say a word (eg: みず), what do you mean? That you want to drink water? You want to give me water? That there is a bottle of water?
And if you only knew grammar and no words... "the ... is ...ing", not very communicational without the words.
At first, it will be hard, as anything will be very different and need to be learnt, but little by little the pieces come together.
You say that you start to understand the grammar lessons. Those come with sample sentences, that's nice as words are in context, that make them easier to remember. There are probably some words there that you start to understand as they came in the grammar schedule several times. You can click on them and add to the word schedule (or make a custom one). At least for me it is easier to learn words I found on sentences first.
I would advocate learning the words alongside. What is important is taking the quizzing situation that you are doing well with (as you described), and say "if I was expected to consume actual Japanese outside of renshuu, would these same things be there to help me?"
It sounds like you have a grasp on the rules and constructs of the grammar you are learning, which is great. However, grammar is just a set of rules, and you'll need to be able to have the ingredients (the words) in your back pocket to be able to take advantage of all that awesome progress (on the grammar end).
Adding a bit of grammar, without overly stressing about it, would probably work wonders for word retention.
I don't know you and try not to make assumptions, but even so, I'd be willing to bet a year's supply of chocolate that you are not actually "not good at remembering vocab". It's more about using words in context and encountering them in different media than about being "good" or "bad" at something (leaving out actual conditions that interfere with memory). Even reduced focus doesn't have to hamper remembering words. Besides, what's the hurry?
My two cents: words are like living creatures, you can keep them alone in individual, empty terrariums, but they'll have a miserable and short life. To thrive and multiply, they need to live together, with rich interactions and a diverse habitat. I can repeat "mi-zu, みず, 水" with five hundred flashcards, it won't stick as well as if I let it live among its peers and acquire some grammar so I can make sentences like "On a hot day, cold water is the best" or "Soda is fine, but drink some water too!"
