@unpossible251 I had a great game last night, so good I even finished it. The spies thing was not the main reason why, things just clicked into place well but Frances flying squad helped me a little at the start and I had lots of wonders but mainly because I finally got the resort strategy right after all this time. What I mean is that when I got Eiffel I already had all my resort sites in place and builders sleeping ready to go. France does help with all of this.
The trouble is you have to pushscience a lot and culture a fair bit too.
But on the spies, at the end I had 3 captured they would not give back and a lot of people disliked me.... I guess I must be playing France historically. Marcel was wonderful but I was aiming for a gold expert, some games just do not work that way. I'll try siphoning to death again another time.
Some Spy advice having had one concentrated run through... Spy experts please correct me if you disagree.
Losses
You will loose a lot of spies at the start and the chances of this happening seem greater than expected but once up and running with a good spy it just rarely gets caught. My advice is keep trying, do not give up... all you need is one great spy to have fun but if you get 3... well you are going to seriously damage an opponent if you concentrate on them.
Target
A target that concentrates elsewhere like Pericles will always use CS cards rather than spy cards so is an easier target while Catherine is unlikely to be worth spying on safely... I found this in other games, my spies often get killed with her.
Target your victory condition competitor, yes we damage spaceports and that's great but last night Pericles was looking dangerous at one stage
but taking a great work off him roughly every 6 turns just made him useless.
For a city target - try not to go for capitals with loads of money if you suspect they have a defensive spy there. The pain of loss is bad enough.
Defense
have one spy on defense duties with the quartermaster promo ASAP. This is your foundation. You can always get a second with seduction but I would either give this to the quartermaster or just leave it. While they often target your capital I now know they do not always and do not always target the highest earner. You cannot cover all your cities so the best bet is to cover your capital and if you get attacked in another place, move your counterspy there. 2 just seem pointless but keeping your cap covered or your spaceport is important so I guess it depends on how important spy defense is.
Promotions
Experience is gained VERY fast for spies and a few successes will lead to master spy quite quickly. the key is the distance to the target city, gathering sources
The documentation states the max spy level is 4 and I can confirm this likely effects your spy ability too. If you look at my master spy choices they all level out at 90% and so the best promotions are not the ones that increase your chances of doing things. They are
Quartermaster for +1 for all spying
Linguist for 25% less time for operations (shaves 2-6 turns off operations allowing you to do much more with a good spy) Combined with the 25% card is powerful.
Ace Driver +4 levels during escape (this +4 levels still confuses me if I can only be level 4)
So get a quartermaster at home ASAP and give your best spy Linguist and ace driver and you are racing.
Expense
Spies are expensive and it is quite an investment but they can be powerful and the damage they will wreak if you do not have them can be quite bad fast. One game I myself lost lots of great works to the point of rage quit. In reality my rage is based on my own stupidity
Fun
Having a good master spy is fun but not enough to engross you as you have to wait for them to finish each task. At best spies are a necessity for a lot of games but mainly not domination. They are a great tool but frustrating because the district list is not accurate and you have to try each city to find what is best and they do not take up much of a game.
Key point
Spies are expensive and using them causes a lot of hate. I will probably only use them defensively in most games initially... so only getting 1 or 2 ... I can always change their use later in the game as needs be.