You could try using IFTTT? I don't think they have direct Substack integration but you can set it up to run from an RSS feed.
Another strategy is to prepare your social posts before you start writing your posts. If you prepare a tweet like:
>"In today's newsletter, I talk about my favourite cakes and the Battenberg that almost killed me."
It can help to focus the mind when you're working on the newsletter, plus you can just hit Send on your tweet when you're ready.
Hey hey :) If you're feeling crafty, you can also check out canva.com and try making your own profile photo / banners. They have TONS of templates and even in the free version you can customize a lot. Canva is my go-to for my Substack and for many other things as well. It's good when you know what you like but don't really have the skills to create it.
You can always move off Substack in the future. It’s a bit of a pain but at the end of the day you own your email list and your content. Some places like Ghost will even help you move everything over. https://ghost.org