If you're looking for a Maastr alternative, Cryo Mix is a good choice. This AI music mixing and mastering tool has tools to improve audio quality, including pitch correction and final mix preparation. It also has an AI Audio Separator for isolating stems from a single audio file, so it's good for both mixing and mastering.
Another option is LANDR, which is more of a one-stop-shop for music production. LANDR offers AI-based mastering, music distribution and a variety of pro-grade plugins and samples. It also supports real-time collaboration and distribution to more than 150 streaming services, so it's a good choice for producers who want a complete solution.
Auphonic is also worth a look, particularly if you're focused on audio post-production. It automates and optimizes audio for a variety of media, including podcasts and videos. With features like an intelligent leveler, noise reduction and loudness specifications, Auphonic is geared toward making your audio look as good as possible with the least amount of effort.
If you want something more specialized, Diktatorial Suite offers instant mastering with text prompts that can be set to high-fidelity standards. It's got features like streaming optimization and options for different genres, so it's a good choice for audio pros who want advanced mastering tools.