There absolutely is a vintage Mac 68K and PowerPC development community, if you allow me to be loose with the word community. To my knowledge, there isn’t a single, centralized space for people discussing and sharing tips and projects for vintage Macs. Instead, there are a lot of different people and mini communities spread all over the place, not all of which are specific to development.

First of all, there’s MARCHintosh and the wider community of vintage Mac retrocomputing enthusiasts, including Ron’s Computer Videos and Mac84.

Next there’s the 68KMLA forums which, despite the name, also include PowerPC Macs for discussion. There’s also the MacRumors forums which have numerous sections for earlier Macs.

Now, for cross-compiling, the main project I’m aware of is Retro68 which supports 68K and PowerPC targets. I’ve never used it though so I can’t vouch for it.

If you’ve never developed software for Mac before, you’re going to want some documentation. Thankfully, there’s a nice library over at Vintage Apple! Also be sure to check out the various collections over at Internet Archive (the filtering options on the left are extremely powerful).

To get more specific you’ll have to provide more specifics about the type of development you’re interested in. Is it Mac OS 9 or X? I assume you have an iMac G3 and you’ll want to run software on it. Have you serviced the machine to avoid damaging it with leaky capacitors or batteries? Have you replaced the mechanical hard drive with an SSD using an IDE/SATA converter?

These are all things to consider if you want to keep your iMac going as a long term hobby!