I'm not a video/film heavy user nor am I a gamer so I don't need the high end processors but expect to get 64GB ram and 2 TB SSD along with the high end video card as I typically keep computers for at least 6 years and believe I won't have a need to upgrade in that time period being a prosumer.
I'm also curious as to what you use your computer for in this case. If you aren't typically editing tons of video or gaming, or composing music, you really don't need all of that RAM and the power of that video card. A regular iMac would seem to offer you, still, more power than you require, still last you many years, and be a fraction of the cost of an iMac Pro. If, just for the sake of "I want a powerful computer to last a long time," that's totally fine, I won't stop you, but you can totally achieve that with the regular iMacs and save a lot of money.
I'm a freelance film composer and audio editor utilizing large orchestral sample libraries and have 40GB RAM in my 2017 27" i7 iMac. I'll sometimes be contracted to produce and edit 4k video as well and I still am not even pushing the iMac very hard. I understand the desire for speed, power, and longevity, but realize you can achieve this with a much cheaper iMac and still have plenty of oopmh to boot. Anyway, just my two cents.
Also, to echo what others have said Re: iMac pro upgrades: extremely unlikely to happen this year, if at all.
Also also >insert the whole shebang about buying if absolutely needing it now, otherwise wait because seems like next update for iMacs in 2019, yadda yadda...<