You’re at the bottom. there are many levels of upgrade from there. the first one is what you propose, an external DAC. It is easy to beat the onboard DAC. But to get a good audiophile DAC you’ll need a few grand. In the budget DAC area, $1k or so, look at Schiit. Then when you have an excellent DAC, you might want to iprove your CD transport. And of course you could spend money on a streamer (another several K) and subscribe to Qobuz and have CD quality by streamping with almost unlimited library.
Long term, the DAC is the heard to a digital system and you can keep upgrading it. If you found $20K that you forgot you had, I would not criticize you for spending it all on a DAC.