Don't have any interior pictures to share right now, just a recommendation. If you ever find yourself in Bath, everybody knows about the abbey but not far from there is a RC church called St John The Evangelist's, it interior knocks the socks off the abbey, well worth a visit and the last time...
What type of photography are you interested in, portraits/weddings are a very different skills set to landscapes or Street photography or wildlife photography.
I have over the years done wildlife and landscape workshops. I find the hands on face to face tuition brilliant.
Choice of film is very much about what you want for the end results, for portraits and pastel landscapes I love Portra, for winter colours etc I love Ektar, that's just a couple of the colour options.
Loads of b&w options too
Agree with Soeren, you are just starting out, the kit 18-55 would be a great place to start for street work, buy a used one, use it for a while and then you can have a look back through the shots you have taken and get a good idea of the focal lengths you have shot at most, then maybe consider a...
The sensor on the D3300 is 1.5x smaller than a full frame so the field of view will be about the same as the figures you have quoted. Choose the lens you want based on the angle of view you want in your camera.
What do you want to photograph with the lens and how much space have you got to play...
Expodisc or gray card for getting it right in camera.
In Photoshop there is a technique for finding neutral grey and sampling it to pick up any colour cast and neutralise it. There may be newer ways to do this too.
Now that just about everyone has a camera of some description those that publish photos have their pick of people who will just give them the images they want, country file calender is a great example, have you read the t&c.
Only the very best ever make it to professional, usually because they...
You would probably be better off with a dedicated Macro lens, loads to choose from, I have always liked the Sigma 105mm. Tubes are handy when you want to reduce the minimum focusing distance of a lens a bit
It's purely for display purposes, and there is every chance a buyer may choose to use a different mount colour. I would always chose to cut the display mount to a readily available frame size to make it easy for a buyer to just buy a frame to drop the whole thing into.
If the minimum focus distance on the 400 is just too long there is only one way to get it closer, fit an auto extension ring (not extender) that will cut back the minimum focus distance at the expense of infinity focus. Kenko do a reasonable set of auto tubes
Focus stacking, fine with plants but for insects you may need to cool them down a bit to keep them still long enough.
The smaller the sensor, the shorter the lens you need for the same framing, that will give you a slightly better DOF, as will using a 50mm macro instead of a 90mm macro
There are lots of ways to package photos to sell but probably the most suitable for everybody is mounted but not framed, everybody will want a frame that matches their decor.
Selling physical prints then you would ideally make up a decent batch and attend craft fairs etc.
Another way, that...
Looking at your image I have to agree with Kendo1, looks like shutter speed, the focus point looks good, the area around the subject looks good. If you want to hit a slightly sweeter spot with the lens try f8.
Keep your shutter speed over 1/500 and preferably 1/1000. Use auto iso if needed to...