For the demanding tasks, I would still recommend building your own as most OEMs will charge hefty premiums for additional RAM, a beefier GPU, etc.
Might as well pick out your own and get exactly what you want.
Nowadays, a high-end Core i7 (especially the newer
Lake generation) + 16GB RAM or more and a GTX1070 or more is all you need.
Make sure your OS lives on a SSD drive + regular hard drive for case archival (ideal would be a regularly backed-up server) and the rest is up to you.
I would select a power supply unit that has at least 20% headroom on top of your calculated power requirements, to allow it to work less hard (last longer) and allow for future expansion.
For the less demanding stations, my personal choice would be either Mac minis (you can install Windows if your software/workflow requires it) or iMacs if budget allows.
They are compact and silent, they look professional and are reliable machines. The cost is a little higher than a low-end PC, but they will last you longer and be more pleasant to use than the compromised hardware most econobox PCs use.
Mac mini is due for a refresh soon, so I would wait for the new model before committing.
I'll save the Apple-haters some time, I will not engage and argue with you, sorry!