Focus on the positives. It helps all of you when you find yourselves stuck in a bit of a negative cycle of lots of nagging, telling off etc.
Buttons in a jar can work well, or stickers on a chart...whatever you choose. Start with a really quick to earn reward so child experiences success straight away then you can start to stretch them out / make them more challenging.
Positive / reward focussed backed up by a very consistent consequence system (we try to use natural consequences as much as possible...my dc are 7 & 5 now & counting still works for them because it has always been 100% consistently applied...I can’t actually remember the last time I needed to count though).
Forced choices & picking your battles also useful / important for sanity saving.
And you’re so right - if you aren’t going to do it, don’t threaten it because all it teaches your dd is that she doesn’t need to worry because mummy & daddy always say that but it never actually happens.
Naming feelings too - hers and yours - can really help keep things calm & good emotional literacy for later on. So your Christmas tree example - ‘aw dd, I can see you’re so excited about seeing your cousins, but this is making me feel really frustrated and sad because I want to see our cousins, so I’m going to go and sit out of this room to calm down. Come and find me when you’re ready to put your trousers on.’ Then ignore her thus removing the game & hopefully motivating her to make the right choice, and massively praising her when she does so.
The language of choice is also really helpful actually so she understands she’s in control - she can make good choices.
Good luck! This is so normal for 3.5 and it does get easier, definitely! But the work you put in now on establishing clear & consistent expectations and boundaries pays dividends in the future.