Last build I did was about 20 years ago and the for the time, research and effort spent I could have paid someone to build a much better pc 😁 I did look at ordering from Chillblast as I've had positive experience with them in the distant past but for this same exact spec they wanted well over 5K! and their cheapest RTX 5090 build is 4k!
Money no object and simply because I like the understated design (and yes even with the compromised performance because of the size) I would have bought the Corsair One but at £6k the wife would have left me and the divorce would have cost me even more 😐
To be fair to PC Specialist, they are trying to fix the issues, so will give them time to come up with a solution, but would have been better if they had properly tested it before sending it out!
It IS a cracking PC for the price mate, 5090 and 9800x3d are top tier, the speed and size of the ram is in the performance sweetpsot and the PSU is more than adequate to run everything fine. But being hyper critical as most armchair PC experts on the internet are, the motherboard could be a bit better, but the consensus is that it's perfectly fine for an 8 core CPU like the 9800x3d, I just wouldn't be confident that it's up to the task of running more cores than that. So in that sense, I think the components in your high end build are more than adequate.
I seem to be all alone on this forum having an Intel PC, simply because I prefer high ram bandwidth (Over 130GB/s), a monolithic die without chiplets and having the system latency under 50ms. The cache of the AMD chips performs well in some games but not in others. and in some games it simply crushes Intel CPU's with higher framerates. As a gamer who is also a ram geek, that's why I went Intel. My old 5800x3d was faster than the Intel in the Assetto Corsa games when combined with tuned ram, I just had some minor performance issues in a couple of games with AMD which made me try the Z790 platform which I've been really happy with.
At the end of the day, high end AMD and Intel SKU's all have their pros and cons. And what you have will serve you well for many many years to come.
I hope PC specialist sort out the niggles you are having. They have been in business for many years and surely know what they are doing.