I am really confused about this. Firstly, it sounds like you're in that place when you are focusing your thoughts on having kids. You need to understand if you want to postpone it, that's your choice, but it will be difficult emotionally - from personal experience. This issue stands out for me, not so much buying clothes which I think is secondary to this.
Secondly, and I don't say this lightly, I too spent a long time at university, taking follow up exams and progressing my career and then got pregnant easily in my 30s and my life seemed picture perfect. Only to have 3 miscarriages in a row and everything coming crashing down. I have my beautiful son now, and I am not saying this to scare you, OP, I am saying this for you to be aware that your wait may be unexpectedly longer than you envisage now. We just don't know these things, ever. As long as you have your eyes open, it's entirely your choice of course.
Baby clothes are not a good investment, let me tell you, my son has a big wardrobe now and I sold some of them, they never sell for anywhere near the original price, even discounted. I buy ahead too on sales, but he is already here and I know what suits him and what brands are actually my favourite too. So, in the nicest possible way, stop buying now.
Finally, you do not need £50k for maternity leave. You really don't. I believe you may be too focused on the sense of security, particularly financial, because of your background. But honestly, no one needs this kind of money for this. If you have it, you're honestly in an extremely good position.
As a professional and a mother I will tell you as well, whilst there are better and worse times career-wise for having a baby, I absolutely believe that if you have your qualifications sorted, you will get where you want to be regardless. In hindsight you'll find that your career may have slowed down a bit because of maternity leave etc. But in a grand scheme of things, you have 30+ years of working ahead of you and no one, absolutely no one will remember whether you took a break at 29 or 35 or both. It doesn't really matter. It might matter if you want to build up a good relationship with your employer to enjoy more flexibility. But in the post-pandemic world things are looking more flexible for many professionals anyway. And the same goes for your partner - who I assume doesn't need to worry about childbearing, so why can't he step up more and provide you with a comfortable environment sooner? (and frankly it sounds you have this environment already, but you don't perceive it this way). You may want to have a word with him and agree your shared priorities. I'm massively career driven, but children are a huge blessing and definitely a priority now. Think about what really matters to you now. xx