1. Need more multitrack features (Audacity is more an editing tool than a mixing one)
2. Need a DSP (Digital Signal Processing) package so you can create your own audio processing patches
As Audacity uses LADSPA plugins, you'll have the same ones in Ardour and any other DAW (Digital Audio Workstation) software. Another DAW would give you other analysis and another UI, but unless it goes beyond LADSPA/LV2, you'll have the same audio processing plugins. A "next step" here would be working with audio directly by programming, designing synthesis models, filters and so on. Usually that's not easy, but that's what many contemporary music composers do all the time.
For the asked "good audio processing/editing/enhancing setup that can run on different platforms", I suggest you try AudioLazy (https://github.com/danilobellini/audiolazy) as part of this setup. It's an open source DSP for Python. Functions like "lowpass", "highpass", "resonator" gives you some common linear filters, and you can make your own [time variant] linear filter with the "z" object, besides basic operations (e.g. multiplying signals), synthesis (ADSR model, table lookup, FM synthesis, etc.), non-linear processing (e.g. getting the "arctan" of a signal to distort it), etc..
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If Audacity is becoming too limited, perhaps you
1. Need more multitrack features (Audacity is more an editing tool than a mixing one)
2. Need a DSP (Digital Signal Processing) package so you can create your own audio processing patches
As Audacity uses LADSPA plugins, you'll have the same ones in Ardour and any other DAW (Digital Audio Workstation) software. Another DAW would give you other analysis and another UI, but unless it goes beyond LADSPA/LV2, you'll have the same audio processing plugins. A "next step" here would be working with audio directly by programming, designing synthesis models, filters and so on. Usually that's not easy, but that's what many contemporary music composers do all the time.
For the asked "good audio processing/editing/enhancing setup that can run on different platforms", I suggest you try AudioLazy (https://github.com/danilobellini/audiolazy) as part of this setup. It's an open source DSP for Python. Functions like "lowpass", "highpass", "resonator" gives you some common linear filters, and you can make your own [time variant] linear filter with the "z" object, besides basic operations (e.g. multiplying signals), synthesis (ADSR model, table lookup, FM synthesis, etc.), non-linear processing (e.g. getting the "arctan" of a signal to distort it), etc..