I’ve been using reading mode for about 10 days now, and I’m really happy with that.
Before I was just translating first hit but now I have 2 additional hours of reading
Personally I still do filling in. I could recognize easily なくてはいけない、なくてはならない、… but the fact I start to be able to write those quickly is because I trained them 
Of course, it means I have the synonym hells. To be honest, I memory the hint → structure, then I just look at what is around (is it a noun preceding ? a verb ?) to infer if I need some nominalization, etc etc, then I write the conjugated answer.
I think it’s quite important to write yourself to fully graps the small differences that make nuances. That’s also how you learn exception the best, for example by failing to recognize godan verbs that looks like ichidan (走る, 走って).
Also, sometimes it’s quite important to know if the past of している is したいる or していた。I think If you just recognize things, those 2 patterns might triggers the same recognition patterns, while in fact not really know example the difference.
Of course, I illustrate with easy examples, but latter there are more and more subtle things that need to be acknowledged/practiced, for example when to put お in humble form, etc
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