@Tesla1856thanks for the 1, 2, 3 breakdown. Realistically, this is what I shall do.
If it came to replacing the CPU, me joking about breaking things aside, I do believe that I could do it - however, even saying that I would still get a professional computer person to do something like that for me. Because if ever there was a genuine chance for me to successfully destroy my computer with the slip of a hand, or applying too much pressure, accidentally thumbing the pins, etc. etc. it is this exact upgrade process. It's an accident waiting to happen with my name written all over it. I may sometimes be a bit careless, but I'm no fool.
Ultimately I have a big sentimental attachment to my R4 and when I imagine new parts, it's always inside my current computer.
If I'm honest with myself and I saved Xthousand(s) of pounds, it makes no sense really to overhaul at great effort and labour the R4 when I could just invest in something completely brand new / current, as opposed to eeking out an additional 2-ish of years of hopefully improved performance from the R4. I totally get that it can be done - like how Cass' friend has done but that takes serious know how and commitment - the know how side of things I am unfortunately lacking in.
Really I think if I left this computer entirely alone (as it is completely stable, like you said) it would be good for arguably another year at best. It's already struggling now or at least feels that way.
What I'm after in the here and now is simply a speed boost. I'm thinking the new SSD will probably blow my mind as I've never used an SSD powered OS on a computer believe it or not.
I do believe a graphics card would give a processing boost to my current build as my most used applications all use GPU acceleration (Photoshop / Illustrator / Acrobat Pro / Premier Pro / After Effects) but if I'm buying a powerful graphics card - I might as well really consider the point of having such a powerful card and pair it with equally current / powerful CPU's / Memory / Motherboard.
By the time I've saved up some serious money, it will be by next year. By which point there should be better RTX support and this years cards should see a nice little price bump down to make way for the 21-series or whatever Nvidia decides to call them.
Ultimately for what I am doing work wise (and as I'm now producing more video / animated content) I should probably be on a Xeon or even Threadripper workstation. But I can currently more than happily get by with high end consumer parts or prosumer parts whatever the kids call them these days.
In terms of how long this has been a video production machine (I don't think 10 years is realistic either) - I've been doing this sort of work commercially now since October 2018 (prior to that noodling around for a couple of years beforehand learning the software) - up until then (2012 - 2017/18) it had been solely graphic design (illustrator, in-design, acrobat), image editing/manipulation (photoshop), web work (dreamweaver).
Now that I'm consistently using Premier Pro and After Effects on lots of projects for the last few months I've noticed a definite "struggle" with this computer at times - especially when the work load is high.
For now though... SSD.