So I may have now reached a partial solution for this.
Those changes to the settings have dropped the general temperature of the GPU JT to about 35c. Which is about 15-20 degrees lower than it was.
Whilst playing the game as a test last night there was no Wattman crash but the JT got up to around 100c. Now I don't necessarily think that's an issue now with the GPU. I think I need to do a general temperature/fan test of the machine and case to check they're working properly.
I've one more week on online teaching.lecturing to get through and then I'll do some temperature checks in game with the case panels off etc to see if it is just an airflow issue now.
But we may have nearly cracked it now!
How to OVERCLOCK and UNDERVOLT RX 5700XT | ADRENALIN 2020 Easy Guide This video is by Ancient Gameplays. It shows you everything I explained except fan control. Apparently the 5700 XT doesn't run a "zero RPM" config but you can increase the base speed and change the curve to ramp up quicker to keep the temps down. The voltage you posted earlier is identical in the video, so your all set that way. Your power limit can go to 50, my 5600 XT only goes to 20. That extra juice is only used as needed by the GPU. 100c is actually "ok", 115c and it will throttle back. He mentions reducing GPU clock to 2000 for best performance/stability, others claim the same. The VRAM OC will work at 1800 but the lofty 1860 very well may not. For me 1800 nets 10-15 FPS, 1860 about 5 more but will "green screen".
Brand matters. If you have a THICCIII XFX card, they run 100-110c. I noticed this guys card in the video runs 63c, I think his is a Gigabyte Windforce RX 5700 XT from other videos. ASUS runs hotter, it just depends. If your fans are at like 60% or more at 50c, that's as good as it gets to hold it back from attaining that 100c. These cards heat up quick. Running with the side off is ok, but doesn't tell the true tale of airflow. General rule is intake front, side, bottom, exhaust top and rear. The idea is to get positive case pressure. So with a tissue or napkin, you can check that by seeing how good they stick to the intake vents and how well the push away from the exhaust. If it just falls off an intake area, there may be a problem. If it is only barely pushing from the exhaust area, you might want more CFM outbound. You may find with the side off the card runs hotter simply because it's lacking the positive pressure of the cool room air and relying solely on it's fans. When I take my side off while running, the fans pick up speed like covering a vacuum hose with your hand and releasing it(not as extreme).
Case matters too. I run a Fractal Design R4 S, top mounted AIO (exhaust), 2 x 140mm front intake fans at 1000 RPM (UPHERE, 49.8 CFM, 25dBA), 1 x 140mm (same brand) side intake. I use the bottom as a intake vent and rear works as an exhaust vent both no fan. Those 3 fans, combined with the AIO's 2 x 140mm PWM's, the GPUI's 2 fans, and 1 built in SB fan pull hard enough that no additional fan in needed. A napkin will stick to all intake areas and the bottom vent but the top it blow the napkin off and the rear it blows it away from the case. Noise is about 25dBA total, 30 when the card gets going but still less than a window AC. Fans blowing in all kinds of direction create a static flow of swirling air that doesn't exchange the warm air inside with cooler room air fast enough. There's a video on a case with 12 fans and a heat issue, lol. It shows what I mean about pressurizing the case but not exchanging air.
So with airflow, the general idea is to fill the case with air and exchange it with room air every 30 seconds or so. Just enough time for the air to carry some heat off the chips/radiator. Too quick and it'll heat up because the air didn't have time to soak some of the heat off the internals. If you have a sensor for internal case temp. it should read about 5-10 warmer than room temp and you're fine.
An over explanation of things you may already know. Hopefully you get something out of it all. At the end if 100c is what the card runs at and works fine, let it be.
So it looks like this might be solved now.
I've had new fans installed in my PC and they're working well. The new power settings mean that the GPU fan is working to deal with the temperature when playing Total War.
The JT is getting up to around 80 degrees ish but, as of yet, no carshes again or anything similar.
It may well be that's the temperature it runs at for that game. If the power settings are dealing with it, and my new fans are circulating the air ok, then it should all be good now.
If that changes I'll return here but for now: thanks everyone!