I decided to just go YOLO and spend most of my money on the keep as soon as it became available. (Currently party levels are 3-4)

I'm curious how it's worked out for those of you who've been playing longer. The ranger's trail resting bonus seems kind of nice, and I get the feeling that I need something like that just to not be fatigued on arrival every time I travel as the campaign map continues to stretch out. But still, blowing 2k gold for unlimited resting bonuses I had been getting by without using at all until now is maybe iffy. Sure it feels good that it's permanent, but on the other hand, the intimidating wall of possible upgrades looks endless, and I don't know how well it scales.

Unless taxation or quest rewards pay off, I've basically given up on buying magical or unique items in store for the forseeable future.

I'm also sort of paranoid about Security. I have no idea how it's balanced, or if 1:1 security:prestige ratios are good enough, or if it just doesn't scale that way. It sounds like you can only know in hindsight. The wiki also informs me that a random bad guest can show up, hang out for a while, and just kick your security level in the shins until they leave. So lolol, looking forward to that.

Has anyone tried it both as a highly managed estate in one run, and as a hidden basic hideaway in another?

Well money is no problem in this game (maybe if you only blaze through the critical path). I've finished White March yesterday and I had over 190.000 cp. With all Caed Nua options maxed of course.

Prestige only says how much tax revenue you'll get. Security is only important if you don't resolve attacks on the keep manually. Something I always did. If the security is really high, the keep can thwart some attacks on it by itself. You'll also loose less tax money to bandits.

I generally love keeps in such games, but Caed Nua isn't really doing it for me. I wish there were quests around that keep, other than the endless paths. I was really into the decisions you had to make for your keep in Baldur's Gate II. When I heard there was such a keep in Pillars, that's what I expected. Upgrading something, just to get a resting bonus isn't as satisfying.

With the keep, for me, time was always more of a problem than money. I could always come up with the cash to do one enhancement, but there was a long time to wait between enhancements. Maybe I'm just too impatient.

Is there any consequence for rest-spamming? I kind of don't like the idea of resting at full health, but if it has no impact, I would start considering it.

Well... A lot of loading screens, busywork and playing the game agains its core design. It's not that there's a consequence for rest spamming, it's that there's never any actual need to do so - uless you're bad at the game, at which case lowering the difficulty is the right option.

Although with the expansion 2.0, the resting bonus improved, now almost everything is +3, or +2 if are combo.

+3 might on every rest is a nice bonus. :-3

Or if you've got a hard chest to open, +3 mechanical skill.