As far as the cameras are concerned, there shouldn't be any significant difference between the X-T3 and X-T30 in terms of image quality because they use the same sensor, processors, and algorithms. The main differences between the two would be in how they handle. The X-T3 is larger with:
- Larger view finder and eye cup.
- Optional battery grip.
- Weather sealing.
- Dual card slots. This is more of a peace of mind feature than necessity. With mirrorless, it's more difficult to not notice a card failure than with DSLR.
Although I have not used X-T# models, I have used X-E2/S, X-T20, and X-H1 fairly extensively. Reviewers seem to consider the X-H1 very different from other X-series cameras, yet I find the differences among these models to be mainly first-impression differences. The biggest shock was coming in from a different system. After that, it's easy enough to switch from one model to another.
I wouldn't necessarily consider the X-T3 "better" than the X-T30 if you don't need the additional features. If you have good eyes and don't need the weather sealing, the X-T#0 cameras are great deals. You get the same image quality and many of the features in a smaller, lighter body.
As for lenses, I have the 18-55/2.8-4. It has OIS, and is very sharp. There's no point worrying about prime vs zoom with this lens as far as sharpness is concerned. The main problem with it is it doesn't extend long enough for my liking. I would prefer 24-70/2.8, but FujiFilm doesn't offer such a lens, so I use adapted lenses. The main reason to go with primes would be for the wider apertures. (I have an XF 35/1.4, as well as many manual primes.)
I don't have the XF 16/1.4, but probably wouldn't like it because I tend to dislike focal lengths wider than about 24-28mm. It also does not have OIS. Whether you would like it is very much personal preference. Consider trying out some different lenses before you buy.
I don't shoot video, but based on YouTube reviews, adapted manual-focus lenses are viable options for video. They're fairly inexpensive, so you can collect a set of old, fast primes to cover a range of shooting needs.