Personally I have a general idea of how grammar works in Spanish, but I do flip through my little Spanish grammar book from time to time. So rarely I would have any problem with the reading comprehension, but I do think it is opportune for me to divert more time to a systematic approach in study of Spanish grammar.
We have no problem understanding the text even if we have a vague idea of how grammar works sometimes, provided there are context and one’s overall sense of what’s happening in the article.
Studying of any grammar rule should boil down to following processes, noticing, deciphering(cracking), assimilating, memorizing, and integrating.
First we notice a grammatical structure in a text, we try to make a sense of it by consulting a reference book or from previous encounter with that particular grammar rule, then we try to understand it and take it to be part of ourselves, and commit it to our long-term memory, and lastly use it freely as part of ourselves.
In my opinion, first three processes are good enough if you devote most of your time in passive learning with listening and reading only, last twos are fundamental if you want to express yourself freely and naturally like a native.
I am working on a book called “Fuentes”, and “Spanish composition”.
I prefer spend more time in writing and response on online forum in Spanish than practicing with sample exercise from the grammar book. I do recommend you to write a draft for your interview in term of questions and answers, answer to any particular question can be elaborated in 3 ~ 5 sentences. You can also use the grammar rule in your sentence which is related to your own circumstance, which helps you learn faster and easier.
Grammar study requires more effort on your part comparing to reading, I do recommend to break it down into two 20~30 minutes sessions if you can devote one hour to grammar study daily.
Just like with reading comprehension and learning of vocabulary words, it is a lengthy process for all grammar rules to be integrated into part of ourselves, so take your time.
PS : Comparing to the thorough approach with a textbook, online Youtube channels for Spanish Grammar are more interesting and really helpful too.