Just FYI, is your concern with tech stealing out of the gate England....or do you just not want tech stealing around Renaissance?
As far as National Wonders,
Printing Press seems reasonable. Makes sense flavorwise, it fits for when spies get going....often not the strongest tech so gives a little more incentive to go for it quickly.
Runner up would be the Hermitage. I don't think its as good a choice but if you want something a little later, but not MUCH later, it fits the bill.
TBH I find the necessity of a 'catch-up' mechanic a bit...blegh and unappealing/avoidable. I don't think spies should be a rubberband, I think it is bad play.
I keep forgetting spies are a thing you need to assign now over and over again. In the old system you only had to do so in spare circumstances like a diplomat fleeing war, city swapping ownership and a spy dying.
It would really help me if assigning a spy would become one of the mandatory end turn actions like vote on world congress or move this unit. Otherwise, it would help me if a mission end would directly open the spy screen again or something. I keep catching myself that there is a (1) or a (2) in the top right indicating spies are not doing anything and it's a real feelsbad.
Also sidenote the dialogue menu has twice the option for poisoning well? Poisoning water supply might be a more futureproof description btw as this doesn't fit modern era for instance.
Twice? Not sure what you mean.
And yeah, I could make spies block end turn, probably. Github post?
G