My advice is that you don't owe any brand any loyalty. You don't have to collect them all, especially not brand new. You don't have to be limited by what's currently in stores or trending online: the surprise of thrifting, or hunting second hand and customising can really add to the fun.
I do recommend setting a budget and spacing out purchases so you have some money aside when you find a 'must have' and have things to look forward to. Remember to budget in a display so you can enjoy looking at your collection.
It really helps to hold a doll in hand, examine it from all angles and see if that special something clicks. Collectors of high end dolls call that 'bonding' - it's that inexplicable piece of design that makes a doll the right fit for you to project ideas, character, even facets of yourself on to. A spark that makes that particular one appeal to your aesthetic and moods. It might be an ugly dollar store doll from the eighties rebodied and given a wig, it might be a 200$ doll, it doesn't matter, if that face cracks you up or speaks to you then it belongs.
Now, what's odd is that feeling can happen for a full collection/brand of dolls or it might only strike twice for that brand and spark again somewhere else entirely different. You might love anime styles in asian dolls and hate the big eyes in western dolls. What sparks joy doesn't always make sense.
So you'll see collections that are all rainbow high or collections that are all over the map in terms of brands but all use goth or neon themes. Others still don't seem to have any cohesive theme and span all sorts of eras but if you look closely the owners personal tastes start to emerge. It's really easy to get caught up in trends and lose sight of personal taste when on social media that prioritises the new releases or having a full set. If you find that energising, great. If not, try to get a sense of when you're starting to feel pushed or like your purchases are making you feel uncomfortable.
It's not basic to enjoy the dolls as they are, displayed on the shelf, either. You don't have to restyle them, play with them, do photo challenges for them to have 'earned' their value: that can be fun but it's not for everyone. They're little pieces of art and people interact with them differently. For some it's a reconnection with childhood and imagination play but for many it's just enjoying colourful and cleverly designed sculptures. I just think they're neat is a valid explanation 💞!