Ryzen Low physics Score

You're not just getting a low physics score, you're also getting a low prime numbers score as you can see from the images posted here. The prime numbers test can be a bit inconsistent (my 1500X has gotten results between 39 and 45), but the physics score shouldn't be (at least not more than a few percent, which is normal). On my CPU the average of five benchmark runs sits at roughly 650 points.

As has already been stated, having just a single stick of RAM is not great. That's what I would fix first. The problem is if you get another single stick of RAM, there's no guarantee it'll work with your current stick of RAM, even if it's the same exact model. This is why RAM should always be bought in kits. Another thing I would do is check that - unless you've already manually set them - the memory timings are what they should be. I'm assuming you're using the RAM's XMP profile, so it should be relatively easy to find out what the timings should be and then see it the timings in the BIOS match.

P.S. There's nothing wrong with having a B350 chipset, as the chipset isn't really a factor in how the CPU performs. The thing that could be a factor is that B350 / B450 boards usually do not have the greatest VRM solution, but your motherboard VRM is just about the best you can get on a B350 board. It's not as good as the best X370 boards, but still, you shouldn't have an issue with the motherboard unless you do crazy overclocks (it's still just 4 phases for the CPU, after all).

cpu temps, mem speed and windows process affect cpu benchmarks

as for GTAV dropping frames is normal is a big area

you should use 3dmark instead to get your physics score and it even shows combined score which would show you something more closer to gaming scenarios 

I've always found this benchmark to be quite inconsistent. OP, you can always try other benchmarks and see what people are getting there.

Also, do you use 1 stick of 8GB RAM or 2x4GB? 1x8GB means single-channel which really is a bad thing in general.

B350 is a really low-tier chipset. Together with old BIOS and bad drivers you can get these bad results.

P.S. why on Earth are you asking in a ASRock forum, if you're using MSI? Not that we won't try to help, but most guys here couldn't care less about MSI.