Writing opinions I may be wrong about - but I don't think so! 

Enjoyable opinions! I was schooled on Thomas Hardy who describes endlessly. I think description is best terse and if it can be used at the same time to add to characterisation, so much the better!

I think creating or using a solid world base is key, having a deep world lore can lead to being able to create many many stories based within this "real" world. Character creation is usually comes for me as a story to story thing, where they impact the world they exist within, so knowledge and events that happen in one story written in the world set 100 years past will impact events within another story set later down the timeline. whether it be how different groups of people treat each other and so on (just like real history which I'm a huge fan of reading about.) I think it helps to have a concise superdoc/wiki that encapsulates for the most part, the world you are going to work within to spawn many stories. Character sheets are useful for keeping track of the base character which you wish to evolve, and perhaps to keep track of where they have gone. Certain details you can always fill in later if you want to simply because certain details like eye color usually aren't the most important thing people will remember about characters in books but can help cement certain key details like scars, special relevant tattoos, perhaps relevant information can be attributed to these details to keep track of certain things like a scar given to a character by another character. (useful if you find yourself having gaps in writing and don't want to come back and lose continuity) I don't think with lore, it should be heavily pushed to the reader early even if there is a lot of lore to be had, you can supplement this by instead shifting certain exposition from dumps to organic dialogue where the reader gets clued into things going on in the world you have built from the words and actions by the characters within the world. (character is passing in a city square and a town crier is overheard detailing some issue going on in the region, the character seeks more details, and that issue becomes relevant later on, or they arrive with some details into town and ask a bartender about XYZ who overheard someone mention something, A boss giving someone an assignment telling them what they want done.) Continuity in writing is key, because if characters change description from one part of the book to another, it can break the immersion, so while the writer may know what the character looks like, describing them can help the reader understand. Useless or over-description is a problem; some authors over-describe the size of breasts, or describe skin in microscopic details. This is a trend to try and avoid as it just bloats the prose. It helps to kind of play on the line between vague in some scenes and detailed descriptions in others where perhaps detail is more important (mental maps for say a fight scene) vs more vague dreamlike descriptive flow in a sort of flash fiction feel for say a tense scary high stakes chase between a vulnerable character and some bad guys; this is how you can convey a sort of confusion onto the reader, by taking away details they can use as "senses" and can get caught up in the moment. I'm what you would call a very strong visualizer mentally so its interesting how similar in style I have to you. Themes and messages for me, I usually just write and sure I may pour some of my own personal beliefs sprinkled throughout the world and story and characters, I stay aways from heavy handed messaging because that usually always turns people away, I do tackle topics people may relate to on various levels, but I never lean one way or the other and leave a lot of that up to the reader to agree or disagree with. Like you said, its best to leave it up to the reader what your underlying message is because that's more important than conveying some secret truth only you know (It sounds bad when put like that.) It's better to view stories (especially fantasy) as a very Personal insight into the heart of the writer without preaching their beliefs onto everyone. Or at least my favourite stories are like that. I very much agree with a lot of what you said.