Making a high level adventure for D&D 5e that is suitable for publishing...
To me the biggest challenges are:
- potential diversity of the party (class, features, optimization, magic, etc)
- diversity of the player's problem solving skills and tactical competence
- DM expectations and abilities
So how do/would I approach these?
- Understanding that the adventure is going to need a section to educate the DM on expectations and how to run the adventure.
- Flexibility in challenges
The first is fairly easy, about all you can do is write a few well written paragraphs on explaining how flexibility is important, that the solutions (even those presented in the adventure) are only examples and guidelines. That the DM may be able to run the adventure as written, BUT that the players may well go in directions not foreseen and therefore adaptability is important.
Now, how do you write an adventure that is flexible? Factions that have motivations is the first step.
This can be several competing factions that the party has to overcome (i.e. Dragon Heist Remix) or a single BBEG that has various resources that they can draw upon as needed. And then the DM needs to adjust what the NPCs do based upon what the players do. (The players scry the location of the BBEG and skip everything? Well it was a well-prepared trap, or...). But, don't punish players for creative solutions and only give the illusion of player agency.
Encounters are often planned with pools of re-usable resources (NPCs) to call upon. i..e pre-create ~10 different groups of (re-usable) adversaries (a set of guards, of scouts, of shock troops, of casters, of...) and then mix and match and pull them into encounters either alone or to supplement the major opponent of some obstacle. This makes the design easy, and flexible, while maintaining a "theme" (i.e. all 10 groups are creature types related to the adventure, cult, BBEG.) Another key feature of this approach is that each encounter can vary in difficulty by letting the DM call in waves; i.e. the location has a set of guards, but they call for help and are soon joined by a party of caster and/or shock troops. Whatever the DM needs to do to provide the appropriate challenge for that encounter.
Which means you have to indicate how challenging each encounter should be in your design and how to adjust it. (i.e. this encounter should be a significant challenge for the party, try to insure they use half of their daily resources here.) You can still do 6-8 encounters/day if that makes sense. Or go with daily nova fights. It all depends on your adventure type and style.
And then remember that a high-level adventure probably should not be about defeating a dracolich in combat. Or just about the combat. But perhaps there are political, social, or strategic objectives that have to be solved before you can actually kill the dracolich. (i.e. maybe the dracolich has a phylactery, and not only do you have to find it, and steal it out from the vaults of the High Captain of Luskan. But then you have to make your way to a source of primordial fire to destroy it. Then you have to pin down the dracolich so it actually stands and fights. Then you get to kill it in combat, all before it raises an army of undead to swarm your beloved city.
Now you have a high level adventure that is flexible. Flexible because the party can solve each step in numerous ways. Flexible because combats can be adjusted based upon the abilities of the players and characters. And it has the content and outline that makes it publishable.