I think that yes, you will need to be proactive. There are systems of support but you’ll need to access them (with their consent). If she is given a diagnosis, perhaps you can have a conversation with her around ‘preparing for the future’.
It’s good they have a cleaner and gardener but if money isn’t an issue, you could look at getting extra help in the form of a ‘PA’, these are people who do all kinds of jobs to help people live independently, such as organising meds, doing shopping or other general jobs.
My dad is elderly and has Parkinson’s and while he doesn’t have dementia, he needs a lot of help. He has carers two times a day plus a ‘PA’ for 3 hours a week. She will do odd jobs or help him pay bills etc. I live about an hour away and I see him every weekend. I do everything else, organise shopping, meds, doctors appointments etc. Between us all, we run his house quite efficiently. At least for now.
It took a while for my dad to accept he needed help but once he did, he didn’t look back. He loves his carers now and they brighten his day.
You might like to consider getting health POA too while she is able to consent. I understand that getting it at a point where she can’t consent is a tougher process.
I think it’s good (if you haven’t already done so) decide for yourself what you can and can’t do. All too often people assume responsibility for ageing parents and run themselves into the ground. I knew from the start that I couldn’t ever do my dad’s personal care (he’d be mortified at that too). I’ve certainly got involved in cleaning up loos and floors if he’s had ‘accidents’ etc but in general I stick to doing his life admin and emotional support. I hope you’re able to access the support you all need.