Yeah there is no hardware acceleration in the 5.1 preview as for a rolling release its important for Manjaro to keep totally mainline.
5.2 sees the introduction of the Panfrost kernel drivers and with Mesa 19.1 some form of hardware acceleration should just work (That is really the 1st destination).
You will not even see an attempt of any form of hardware acceleration in 5.1 from the Manjaro preview, but you can not compare the Radxa specific hugely patched 4.4 version with custom packages as a like to Like.
You can check the lxqt version of 5.1 as a comparison and think it is similar but lxqt still has quite a way to go before it feels complete IMHO.
5.2 is a sort of watershed for Arm as the Panfrost & Lima KMS drivers means the closed sources Arm binary blobs can be dropped and further kernel versions will see optimisation and function additions.
So many new boards have been ‘working’ but stalled @ integrating the Arm Mali blobs into kernel revisions that its been a real pain.
For Rockchip the patches in 5.3 for HDMI audio & WiFI/BT prob with a bit of arm twisting will get backported to 5.2.
So mainline u-boot & mainline kernel is as good as here, if still rather bleeding edge as 4.4 for some is getting quite aged.
So probably we will see the availability of many distro’s great rolling releases like Arch / Manjaro and scheduled releases like Ubuntu 19.10 (5.3) in October.
I am massively happy that Manjaro is looking at the RK3399 as I purchased the RockPi4 to do exactly the same as being a big Arch fan I have been eager to see an ALARM version.
So many times I find myself on the Arch Wiki as my definitive source of info it would be disingenuous not to be an Arch fan boy.
But Manjaro does come in many flavours Install Desktop Environments - Manjaro and from Mate, Lxqt, KDE the RK3399 with Mali-T860MP4 should give perfectly adequate results as we are talking a pretty big step up from what many with Raspberry experience are used to.
I am looking at your Iozone scores mouth open with a WTF look at the speed of that EVO 970!!! 
I am the same when it comes to desktops, but it was Panfrost, Rockchip, Mainline all coming together in £50 SBC producing real world entry level computing from desktop to micro server, has been really interesting.
Hence why I have been trying m.2 sata to m.2 usb3.0 cards even have a M.2 to PCIex4 to try yet and its all just been curiosity more than application.
I have a rockpi4 in a sad state of dead pcie bridge and now sd card reader after dropping it that is a prime candidate for some OC play and dtbo mangling, just to see what it will push too, but the Arm boards really are starting to cover a spectrum of possible solutions.