it helps to start with an underserved niche

There are many different flavors of freelancers. I try to focus on serving one client well, but there are others who serve multiple clients, often tracking hours on multiple clients each week. I think they are more your target market.

I disagree. I focus on serving one client at a time, as well. Occasionally two, if one doesn't have enough work for me. I have a custom portal I built for tracking time/expenses, generating Stripe invoices, monitoring client budget, etc. Some clients want per-hour billing, some daily. Some want individual line-items (like a git commit, explaining exactly what I did with that time), and some don't care. If this project existed before I built that, I'd be using this project -- hands down.

Sadly, my portal already exists, and is deeply connected to Stripe and my way of doing business. So, I don't ever plan on doing what this person is doing. But I do highly suggest integrating it with Stripe Connect and make the project entirely free, and only charge a % of invoices total (like 1-5%). If it is free, people will use it. If they can send an invoice and automatically charge a client (assuming the client saved their billing information), that is money in the bank and you don't have to worry about collecting or them paying late ... that is worth it's weight in gold, as a contractor.

Moxie is an AIO app for freelancers. Check it out.

I'm a freelancer now and similarly to OP, I'm not sure I want to add any additional costs so paying for your app is a no-go for me when I can use Google Sheets for everything I need.

Basically, I don't see a value-add (YET) for paying for something when right now this admin work isn't costing me any significant time or causing me issues.

As a freelancer, I use Moneybird[0] for this purpose. There is a free tier up to 4 invoices, 10 incoming invoices and 10 bank transactions per month. Most months I fit in the free tier, some months I pay for 1 month of subscription.

Maybe a model like this would work for you as well? The users you are replying to might switch and sometimes pay if they have a busy month.

[0]: https://www.moneybird.com/