First off, if you're looking for music making software with a mobile UI on Linux à la GarageBand on iOS then you're going to be disappointed, the DAW selection for Linux is sparse as is. That said, if you're thinking of plugging in your MIDI controller then you're going to have to use the dock for its USB ports anyway, so the lack of mobile UI shouldn't bother you that much. A preliminary warning for anyone wanting to do any kind of pro audio work on Linux: forget about the vast majority of plugins, virtual instruments, synths or proprietary sound banks (think FabFilter, UAD, Kontakt, Serum), none of them support Linux.
The only DAWs that you can really take into consideration are Reaper, Ardour and LMMS, out of which the latter two are free software (GPLv2+). While Bitwig does have Linux support, I don't think they have ARM builds available. Since you mention MIDI, samplers, sequencers and synths, your only real option left is LMMS. While both Reaper and Ardour have MIDI editing capabilities, they don't have any built in synthesizers and the selection of Linux plugin synths is anemic to say the least. LMMS is an electronic music production DAW in concept (akin to Ableton) while Ardour and Reaper are best for editing and mixing recorded audio (Pro Tools).
I encourage you to try out LMMS on your computer, it's also available for Windows and OS X (no MacVST support, unfortunately), see if it fits your needs. In all honesty though, even if you get it running on the PinePhone, I wouldn't expect it to be a pleasant experience given the PinePhone's general lack of computing power.
Update: Out of curiosity, I took my PinePhone and installed LMMS on it. Not only was the installation quick and painless, it actually works quite well in docked mode on a computer monitor. What's more, using Phosh's "scale-to-fit" made it possible to use it undocked, although the controls were still small and not very practical for a small touchscreen.
The only DAWs that you can really take into consideration are Reaper, Ardour and LMMS, out of which the latter two are free software (GPLv2+). While Bitwig does have Linux support, I don't think they have ARM builds available. Since you mention MIDI, samplers, sequencers and synths, your only real option left is LMMS. While both Reaper and Ardour have MIDI editing capabilities, they don't have any built in synthesizers and the selection of Linux plugin synths is anemic to say the least. LMMS is an electronic music production DAW in concept (akin to Ableton) while Ardour and Reaper are best for editing and mixing recorded audio (Pro Tools).
I encourage you to try out LMMS on your computer, it's also available for Windows and OS X (no MacVST support, unfortunately), see if it fits your needs. In all honesty though, even if you get it running on the PinePhone, I wouldn't expect it to be a pleasant experience given the PinePhone's general lack of computing power.
Update: Out of curiosity, I took my PinePhone and installed LMMS on it. Not only was the installation quick and painless, it actually works quite well in docked mode on a computer monitor. What's more, using Phosh's "scale-to-fit" made it possible to use it undocked, although the controls were still small and not very practical for a small touchscreen.