I have the base model with 8GB. RAM does make a bit of difference, because when I upgraded my 2018 Mac mini from stock 8 GB to 32 GB, there were improvements, but it was still sludgy when using it. Although Mini is a 630. So, perhaps base MBP 645 with 16 GB will be an improvement.
Frankly, I don't trust what they say they can support. Because my Mini can support 3 x 4K or a 5K plus a 4K. Sure it can in the sense that they are all powered and they work, but it's not nice using them.
The only other thing I can think of is different brands of monitors? Because using a BENQ *seemed* worse than a Samsung. Perhaps the Apple/LG would work better.