Prohibition sign showing only after 'restart' option

Hi people. I hope someone can shed some light on this as I'm at a stage now of having no hair left due to this niggle. I've searched 99.9% of forums looking for answers, but only find similar issues that don't fully relate to the issue I'm seeing so here goes.

I've just installed a new battery, 8 gig of ram and a new Samsung 1TB SSD into a friends 2010 13" MacBook Pro. I've done similar fixes on other MacBooks and never really had any issues, though this is the oldest Mac I have fixed....or attempted may be more accurate :)

I was installing High Sierra from recovery but I came across an issue after it restarted to complete the install - I had the dreaded prohibition sign. So I held the power button to shut it down, then powered it up again and it fired up lovely and fast and continued with the install. All looked fine, so I continued with the set up running the updates from the App Store and after the 1st update I noticed the same issue. Each time the system tried an auto restart following an update, I was greeted with the prohibition sign. But, I'm always able to power it off and boot it back up and it would work fine and continue with the update.

After a bit of trouble shooting, I have found I only get this issue if I click 'restart' or if the system is doing an auto restart following a install. The issue does not surface if I click shut down and then power up from the button. I tried this numerous times and I didn't get the prohibition sign at all....but as soon as I try a 'restart' - the sign appears on boot.

Things I have tried...

I tried reinstalling the OS from a time machine backup from their old hard drive - same error showing after 'restart', but works fine on a shut down/ power up

I have removed the kernel files as advised on another thread from /library/extensions - same issue shows on restart

I have rolled back the OS and installed El Capitan from a bootable USB - same issue shows on restart, but again - it boots up fine on a shut down/power up

I've lost count how many times I have tried re installing High Sierra - issue still pops up after restarts

First aid checks in Disk utility came back fine - no errors

I've ran Verbose mode on normal boot and it flew through with no errors, but when I ran it after clicking 'restart', it was hanging on 'HID: Legacy shim 2' and it would show 5 lines of this, then produce the prohibition sign after a short while of hanging. I have nothing externally connected, nothing in the usb slots (which was mentioned on another thread as a possible cause).

Unfortunately I can't run the Apple diagnostic check - I don't think it exists on the Mac now I installed a new HD.

So to me there is something it's not happy with in the 'restart' script. Any ideas?

Just to repeat - the Mac is running like new from power up. I'm only seeing this prohibition error issue when I try a 'restart' from the menu, or from an auto restart following system updates.

I could just say it's fine and advise my friend not use the restart option if it can be helped, but it's eating away at me after 4 nights of trying to fix this so if anyone has a bit more trouble shooting to try - I'll be grateful

Thanks in advance!