Do you use test driven development for your indie projects?

TDD in software is a must, not an option. As well as documentation.

I do test, mostly after writing a feature, which is not TDD.

I do it because of the amount of uncertainty, but once I am sure about a certain feature, then I write tests to give me confidence once I comeback later to refactor it or change it for some reason.

I think you should always write tests for your core functionality, not necessarily do TDD.

Personally for a long time I wasn't keen on writing tests as I thought it would be better to just crank out features (I always told myself I'd write tests eventually, after this feature...). But then I was always paranoid about breaking the login or signup functionality so I made a new account after every... single... update... just to make sure it still works...

So eventually I transitioned to writing tests for the basics, so at least I don't have to worry about my signup being broken and losing potential users. Or the app not working at all.

You can write tests, you just have to be careful to balance it with moving the app forward with more features so writing tests doesn't eat up all your time. Manual testing is fine, but once you have a certain feature set it can get overwhelming and you need to automate it.

TBH, for personal projects, I don't get that much time to write proper test cases (unit/integration). I usually do manual testing.

But If I am building something professionally or for my own startup, mostly, I do.

For me, the reason behind writing test cases after building a feature is, that it helps me catch any kind of changes that were done unknowingly. As a solo developer, I tend to work on a lot of files at the same time while doing R&D (initial phase).
But again, it really depends on if you have got that kind of time. Because in my experience, writing test cases can take a lot of time. Sometimes, more than the time it took to develop that particular feature.

If you are starting up with writing test cases, do it for critical services first e.g. Auth, Payments, Emails (if email delivery is important).
Then, once you have some time, you can do it for others.

I do write tests, which in itself was a controversial topic among indiehackers, but I don't do TDD. Unless you're very clearly defining everything up-front, TDD isn't really practical. I don't see that happening as a solo founder. You need to be super efficient.

Not for personal, but try to do it for clients project