Christiaan, sorry to hear about the issues you've been having. It won't help at all to know, but I had dreadful service on all levels from GiffGaff in the UK; and have been almost unable to fault Vodafone here in Portugal... so these things can be somewhat random.
However, pretty much all mobile stuff here is geared to a less flexible model - either contracts, or PAYG which tends to require a bit more input than you can get away with in the UK. NOWO is the only independent MVNO here, in that the others are subsidiaries of the main three operators, with relatively little separation in meaningful terms. They don't just use their networks, they're the same business. UZO pretends not to be - but this is just for form, it's integrated with MEO. Other players have briefly popped up, but this is a small market.
I can find no updates re the MásMóvil Vodafone merger itself (rather than reaction to rumours) since early March. It seems driven by a Spanish merger which may or may not then roll in Vodafone PT. I am not sure you can make any plans based on it, one way or the other.
In practice, the primary consideration in Portugal for mobile (including mobile data, if living outside fibre areas) is coverage. There are huge variations in coverage/signal strength in much of Portugal. Relevant links to check this are in this thread:
In price terms, each of the main three operators runs offers, but there's overall very little to choose between them on price most of the time.
In service terms - speaking from personal experience, and what others have told me, too - you'll get much further on most things by going into a company-owned store (as opposed to a franchise, where they generally couldn't care less about post-sales service, and I've basically been missold and overcharged by franchise places in the past...). Over the phone, and online, there is disinterest, and even with good Portuguese. (The solution to problems in Portugal is usually most likely to be found face to face, then by phone, rather than by email or online - with some exceptions)
I've used three different Vodafone company stores, and they've taken the time to sort things, often spending ages on the phone to the relevant departments. When I was initially shopping around, they were more helpful than MEO or NOS. So, you may want to go to your nearest Vodafone company store (which will be in a larger town centre unit, or a major shopping centre (not a small shop unit or in a supermarket concourse, generally), and speak nicely to the staff there... and then check network coverage for places you need to connect, and see from there who you want to go with.
Sorry not to be able to direct you to quite what you're looking for!