To be frank, the answer to your inquiry depends on your needs. If I was in your shoes - having your gear, but my needs - I would choose some characterful mono synth.
For example, I am collecting right now portable setup from all my current gear, to have one portable setup in addition to two different studios that I have now. And that setup will contain at least Digitone and Syntakt, probably also Octatrack to mix and mangle them, but right now I am exercising and learning them both via Overbridge. I will probably suplement it with Modal 001, Behringer K-2 and/or Neutron. My needs with that setup are to perform both goa and psybient.
I wouldn’t go with Peak unless playing it traditionally via keyboard or with DAW. I have Summit and I like it very much, but - with my taste - it needs to be played from keys, otherwise I would look elsewhere. I would encourage you to look at the way that you want to play it to get the most from any poly synth. But that’s my take - I sometimes play my OB-6 and Prologue with sequences input with one finger so that’s that. But then I am obviously underutilising them. So that’s my general take on poly synth.
Regarding character mono synths, I think that there are some good advices here. K-2 and other semi modular synths would give you new flavours. You have to think, if you can live without presets. I would add B’s Neutron to consideration, as it has plenty on board (plus dirty delay) and different character than K-2.
If you need presets… There was SE-02 mentioned. So it has presets, it has great sound, and delay! I will add that Modal 001 as a novelty - it goes where most synths cannot go, it has unique and (oh boy!) organic sound. And has that special thing that it is bi timbral and each voice is hard panned, so one can use different presets per channel. In that setup it is great to play via sequencer for either layered mono voices or as two mono synths. I could talk a lot about Modal 001, but it’s not time nor place for that. Just giving you some food for thought.
I will reserve new paragraph for A4. My brain has similar wiring with @m0ld in this regard. I have A4 and RYTM and they have somehow different workflow than my Digitone and Syntakt (I don’t have Digitakt… yet!). This is Elektron and this is Elektron, but some differences give different expectations and that, while being in flow, can actually derail my process. But A4 is also unique synth, with possibility to pass one voice into another might get one thinking about doing multi peak sounds, format like experiments, with SVF and then another one and then low pass ladder, just using two voices, this idea can be executed in stereo! I actually have A4 MK2 and A4 Keys. And while I always prefer keyboards to desktops, I actually feel that MK2 sounds better than Keys. And my Keys is ready to be sold. Although… I will miss its perfect form.
Probably that’s it from me.
I wish you to be happy and fulfilled with your decision.
Cordially!