With permission from all authors involved, here's a patch providing a bridge between Better Drumlin Diner by InstantHero/Vegawagon350 and Drumlin Diner Settlement by Lemecc.
The former mod is a great mod for environmental storytelling and immersion, showing a lived-in and built-up Drumlin Diner. The latter mod is a robust but otherwise vanilla-aligned (read: minimal objects added!) settlement mod. By their powers combined however, the Sole Survivor can choose to help or hinder this small community in an immersive way and build to their heart's content!

Under normal circumstances, if you try to load these two mods together, at minimum you'll have a bunch of meshes all stuck in each other at the Drumlin Diner location - a real mess. If you have Better Drumlin Diner loading after Drumlin Diner Settlement though, you'll also end up with a permanently very broken workshop that will require a hard revert to a clean save due to location and cell changes. That's no good, right? Time to make these two mods into best friends.

Also, it's genuinely difficult to read the name "Drumlin Diner" this many times, so from hereon the two mods are referred to as BDD (Better Drumlin Diner) and DDS (Drumlin Diner Settlement).

This is a long description, but I really highly recommend reading it. It answers plenty of questions I can foresee coming up.

What this patch does:
Makes BDD and DDS seamlessly integrate with each other.
You'll get the immersive quest benefits of DDS (Needing to complete Order Up to unlock the settlement, named settlers depending on your quest choices, cell never resetting and therefore all items stored are saved, etc), with the more in-depth environment and added content of BDD (new navmesh, flavour items and storytelling, new vendor, new structures).

All BDD workbenches are hooked up to the DDS workshop, and Becky and the Brahmin are also now part of the workshop. Almost all cell edits by BDD are prioritised, but all necessary settlement edits, plus Trudy/Wolfgang/Patrick/Simone's special work furniture from DDS are allowed to work as they should.
Where BDD does not use workshop items, these have been replaced with workshop versions in order to correctly register with the workshop. Some BDD items have been moved to allow for DDS to work as intended, and similarly many DDS items have been disabled to allow BDD to work as intended.

Both mods needed to make quite big changes in order to play nice with each other, which is why this is its own page rather than some small optional file bundled over on DDS; for easy commenting/bug reporting/etc. If you do encounter a problem, please first ensure you have adhered to the load order, placed the mods as close to the end of your load list as possible, and not installed any mods that change precombines, as per BDD's requirements. All requirements from the original mods still apply.

For a full-ish list of all the changes to accommodate both mods, check the changelog.

Important notes to keep in mind:

• There's a lot of scripts firing here, especially if you rush to this location or have other intensive things going on in your saves. It's very likely that all five NPCs (six, technically, counting the Brahmin) will need a few minutes to get their pathing sorted once added to the workshop. Try to let them get on with it for a bit, or leave and come back. If you mouse over them in build mode and they're all shown as assigned to something, then they're working correctly. If they're not assigned, something has gone wrong - make sure you follow the load order and warnings below.
• BDD contains a brand new navmesh so that settlers can sandbox and access all the new structures fully. As the area was never meant to be a player settlement, this does mean that if you tear those buildings down, it's likely that NPCs will still try to access areas they can no longer get to. I'm not willing to make an adjusted navmesh with precuts at this time, I'm sorry! But I think it's safe to assume that if you want to use these two mods together, it's because you want those BDD structures and navmeshes to stay.
• As BDD contains a lot of custom objects, I've added mod-specific scrap recipes for those (for everything else, use Scrap Just A Bit More Stuff!), but some items I had no choice but to give you some odd components for in return. You just have to imagine it as though you're "selling" the collection of items you've scrapped.
  

Load order:
Better Drumlin Diner
Drumlin Diner Settlement
Better Drumlin Settlement Patch

Requirements:
Better Drumlin Diner 3.0.1
Drumlin Diner Settlement

Installation:
For best results, install all three simultaneously. If you have previously installed either BDD or DDS and visited any of the cells that would load the Diner cells, you've likely baked at least some stuff into your save, and it's a gamble whether you'll be able to get the cell to reset (particularly since a workshop will flag it to NOT reset...). If you really want to try this, then drop a hard save before installing. You may also need to do the whole wait-inside-an-interior-cell-for-30-days thing.

Basically, I wouldn't recommend using this on a playthrough that is already using one mod or the other. The two base mods are fundamentally very incompatible with each other. Best results will come from using these mods fresh and at the same time. By all means give it a shot, but be ready to revert to a previous save if you load in and the cells are conflicting.

Recommendations:
Scrap Just A Bit More Stuff <- The comprehensive scrap mod without cell/NPC/INI/item edits!

Credits:
Many thanks to InstantHero/Vegawagon350 for the original mod and 4EstGimp for the current upload, and of course thanks to both for permissions.