As you've noted, there is no consistency to the Design Hotels experience via Marriott. Some hotels do the absolute bare minimum required (which is almost nothing) whilst others view Marriott elite members as attractive guests and so offer additional benefits not required under the programme to entice you to return. As a broader point, I find many Spanish chain hotels in particular are extremely reluctant to provide loyalty benefits if not contractually required to do so and view this as 'giving things away for free'. Outside of brands that are part of major international chains (Marriott, IHG etc.), many NH Hotels (probably the biggest domestic Spanish chain) go out of their way to not provide anything more than you've paid for, e.g. trying to charge for late check out, refusing to upgrade even one category, etc.
Mykonian Kyma is a very well run property (proper champagne for breakfast!), I've stayed a couple of times and it seems a bit of a generous exception to the typical Design Hotels approach - I have even been upgrade to a suite with a private pool from an entry level room (albeit during COVID).