If that 32-bit app has a Windows version, you could run it on current macOS using CrossOver. Performance might take a hit depending on what you're doing, but the MacBook Pro M1 runs Windows games fairly well in CrossOver. Wine might also work.

If the app only available for 32-bit macOS, I suppose your remaining options are running Mojave in a local VM, or in the cloud (AWS offers Mojave instances for example) for your freelance work.

Out of curiosity, is this an internal enterprise app, or a consumer app? Most consumer apps have alternatives for 64-bit macOS.

> I know Mojave won't be supported forever.

It's unlikely to receive further security updates at this point.

If you're not on the latest macOS, you're not getting all the security updates. You will still get many security updates if you're one version prior (Big Sur right now), and if you're two versions prior you might get a few updates (Catalina). But you're unlikely to get updates to Mojave after this year.