I highly recommend https://pcpartpicker.com/ and http://www.userbenchmark.com/ for building computers with. They aren't perfect for ensuring compatibility and performance for you computer, but they are a huge help for doing the research.
My MicroATX AB350M Pro4 has two M.2 slots on it, but I will note that only one of them is a PCI-e slot; the second one shares a bus with the SATA 3 port and in some motherboards the PCI-e sockets share bandwidth, so you're generally best avoiding that for best performance. I wouldn't recommend my Mobo for high end audio or video recording because it seems that like some other motherboards I've tried in the past (mainly Gigabyte) the motherboard has ground noise in the audio recording channels independent of microphone noise pickups (This explains the basic problem that effects non-professional-audio-quality mobos Article: Fix front panel audio EMI noise | TechPowerUp Forums; it is due to improper grounding isolation between components); it might be okay for post-processing if you're recording entirely through independent AV recording equipment and saving it to the computer.
At a glance, Biostar X370GTN looks like one of the rare candidates at the moment, but you're going to want some evidence demonstrating that the motherboard doesn't have significant ground noise to the audio channels. Best sources for finding out that would be reading through professional audio recording and engineering reviewers.
I got the Steelwing Enermax case, and I am for the most part happy with it, but I don't think I'd recommend it for your purposes. I tried to build a light computer, but my computer is probably around 10Kg (approx 20lbs), and it lacks carrying conveniences which would properly classify it as "portable" in my book. I have a microATX Mobo in it, but it is really designed for MiniITX. I went with it mainly because it is an aluminum case rather than a steel case. So the combo of case and mobo introduces intolerable noise to recording in excess of what I'd get for just a noise-y Mobo.
Aluminum has nice heat conduction characteristics compared to steel, but the cases tend to be more expensive and harder to find. I didn't particularly want a 5.25" bay or any spinning media, but you might. Also, my case front panel likely doesn't have strict ground isolation between the USB, headphones, power button, LED, and microphone input.
Speaking from experience, if you are going to go with a case that requires an SFX power supply then beware if you want to have a low power profile and want 80 plus Titanium quality. The only 80+Ti SFX power supply I could find was 800 Watts minimum. The Silverstone SFX-L 800 Watt 80+Ti PSU serves my purposes, but it might be spendy and excessive to you. The PSU can also be a major contributor to Mobo ground noise in audio recording.
No Socket AM4 board has dual M.2 slots, you need Threadripper for that.
Couldn't you use a M.2 PCIe card for the second M.2 slot?