Good that you opened this. I wanted to add this topic in a more specific way in the guide as well, but never came to it. Posting in a thread is easier than writing an article for me. Maybe let me give you an insight of how I think about this as QAT? What I wrote might sound a bit like this is the absolute way, but it's just my own opinion about spreads; how I approach them and what my experience I gained over the last 5 years tells me.
The topic spread is actually not as subjective as one might think. You could even say it is pretty logic if you are used to music and mapping because, spreads are not a thing of fortune but of
consistency and the
way music works. I think you can even say that the consistency I am talking about is ruled by the way music works. So, let me start with the first thing I personally think of that should be taken into account for having consistency in your spread(I assume at this point that mapper knows how Kantan, Futsuu, Muzukashii and Oni is mapped):
Representive parts in a song:
When mapping, you have to take care of how a song is made up and map to its parts and their intensity. Because if you follow this, you will have it pretty easy to bring up a consistent structure among all your maps, since you applied in that respect the same idea to all diffs, which leads automatically to a well spread. That would mean, if for example in your Oni, you have more patterns in the kiai part than in your normal parts, and the least patterns in calm parts, you should do
pretty much the same in Muzukashii, Futsuu and Kantan to acchieve a well spreaded set. That's actually the whole magic about spreads and how they work. Though, there is something else that has an great impact on the spread, but which referes to the difficulty of a map in specific, too:
Rest moments:
Missing rest moments are the
most common disqualify reason in Taiko because, even though they seem to be diff specific, they still have a great impact on the spread in general if used inconsistent among the maps in a set. The thing is, while the idea of rest moments or breaks is to give mapper the chance to prepare themself for upcoming patterns; lowering the difficulty in your map and having some emphasition with this, there is still the danger that these breaks appear inconsistent among all difficulties. As a mapper I say, it is absolutely important to have in a map a good relation between patterns and breaks,
which determinates the density of a beatmap. And that's where it starts to become spread-wise a very big problem.
As I said before, the way you approach to a song's parts should be pretty much the same among all diffs. And the same goes for the use of rest moments and breaks. If you use for example in Kantan and Muzukashii a sufficient amount of breaks, you can't have none in Futsuu. To have a picture of what "sufficient" means, I will give a thumb of rule I always use when checking difficulties:
Is the main snapping 2/1 + 1/1 the diff should provide
4/1 breaks (usually Kantan)
Is the main snapping 1/1 + 1/2 the diff should provide
2/1 breaks(and some 4/1) (usually Futsuu)
Is the main snapping 1/1 + 1/2 + 1/4, so in a more denser way, use
3/4 and 2/1 breaks (usually Muzukashii)
Is the main snapping 1/2 + 1/4 the diff should provide
1/1 breaks (usually Oni / Inner / Ura)
(above Oni level, we normaly have no problem if breaks and stuff differs. There you can go as you like as long as it sounds good of course)It's not exactly like that, since it also varriates from case to case and you can have of course different breaks in a map. But this gives an idea how you can approach breaks in a map. The main idea is to look at the main snappig your difficulty provides with the result of having the needed breaks.
Of course, the difficulty of patterns are also considered in spreads. Like if they are for example monotonically or not. But as far as I can see, that actually works pretty well in our community and problem rather lies on where rest moments are placed and how the parts in a song are mapped. And maybe some of you might have noticed,
that these two things are pretty much connected to each other, since rest moments and breaks are the one, who determined (beside the patterns) how dense certain parts become. So the tl;dr output would be following:
tl;dr: To acchieve having a well spread in a map-set, take care of the paste in a song and map to it in a consitent way among all diffs. Additional to this, put a sufficient amount of breaks among all diffs, which at the same time, determinate the density of those different pastes in a song.
Hm. Not sure if I answered everything you asked above. Maybe I forgot something. Was rather a general thing I wrote, huh?