I have atempted many different things to try and get them spawning; I adjusted the config, regenerated the configs, removed mods that could be limiting their spawn and nothing.

I am trying to remedy a fix through the use of a custom scrip,t but whilst doing that, using structureGel API, I was able to visualize the shipwreck structure and find something that may be the culprit to this issue.

it may not seem like much but just from looking at this I know what the issue is: the spawnbox around it is not big enough to allow for proper spawns within the sctucture (assuming that's how it does the check for it to spawn)

To show this, I will use a pillager outpost structure as an example:

The green represents generated sections of the sctucture (modulated though jigsaws) the white border represents the bounds of the structure's area. Pillagers will spawn within the white box encompassing the outpost as that is the area in where they will have natural spawns

if the Skelewag spawns simular to how pillagers spawn naturally within a sctuture, then the reason why its not spawning is because the bounds (white) are the same size as the structure and because of how big they are they will be unable to spawn as a result of them failing to pass any other spawn checks. (primarily if they are capable of fitting without spawning inside another block)