@Adamanthys
That’s excellent, thanks very much for the detailed reply, helpful tips, and useful information!
Since we last touched base, I’ve done a little more play testing, focusing on Zane’s Cooldown, and Adrenaline in particular. FWIW, I thought I’d share my not-overly-scientific findings below.
With no way to deactivate Guardian Ranks, I was stuck with using a ~10% GR Cooldown Reduction buff throughout, but I did some uptime testing in Sanctuary, with and without skill points/gear; and then some gameplay through Lectro City, Athenas, and parts of Eden 6 for anecdotal purposes.
In short, and if maintaining full shields, I found 5 points in Adrenaline (with no other Cooldown buffs) gave me very comparable uptime to running a +20% Cooldown relic. Which is great, because it seems the math there all works out as it should. 
So the choice seems to fall between using 5 skill points on Adrenaline, or using your sole relic slot, to get that extra +20% Cooldown buff. Either way, +30% total Cooldown Reduction seems to do the job, and both options are completely viable for our purposes. 
The big advantage to using Adrenaline would be keeping your relic slot open for better options. I haven’t had much luck with finding decent relics so far myself, but others do seem to have found some with great buffs and utility.
Alternatively, the benefit from using a Cooldown relic, would be adding those 5 skill points elsewhere, and not having to relying on full shields as much. The caveat with the extra skill points being, that you can only spend them on Hearty Stock, or Ready For Action, anyhow.
As you rightly suggest, I think this choice really comes down to what type of shield you are using. Some won’t see much gameplay benefit from the buffed Shield Rate/Delay of Ready For Action.
In my case however, using the Re-Router amp shield, I do get some solid utility out of improved shield Rate in particular. The L50 Re-Router has a fairly low base Delay anyhow (2.9s), but the improved Rate helps quite a lot. Once recharging starts, It cuts the time taken to return to full shields from ~7 seconds to ~5.5, meaning more frequent amp shots if nothing else.
Not really a scientific analysis here, more anecdotal, but thought I’d share anyhow. 