Going to update this with my own experiences about Serbia. I'll be in Croatia again soon. This is my own experience as well as the general consensus with the forum members I met in Belgrade, all cool dudes.
1 Ukraine is better ROI hands down. In short, girls just like meeting foreigners and eventually like fucking them. Serbs have a village mentality that makes dating hard. However, once you have a date in Serbia, you are in the clear. Opposite of Ukraine in many ways with the lizards mentality. Ukrainians will lead you on but you'll eventually be fine. Serbs ,when they like you they like you. There's also just not enough volume for decent day game in Belgrade compared to Kiev. I got two numbers from stunners ,the hotter one(a 8.5) who was eye fucking me like crazy ,neither translated to dates. Even though initial text exchanges were good. One weekend in Novi Sad a solid 9 also gave me crazy iois but her body language became much more cold when her other friends came , some of them beta Serbs who no one was fucking surely. This is the paradox of Serbia. In my experience, especially for high quality, night game is basically useless in Serbia. You can get a few passive aggressive makeouts from hotties who won't fuck or date you(just friendzone). I didn't like kissing girls because they seemed too distracted due to all their friends being present. Their minds and tongues weren't into the experience
2 Skip spring. The last few years spring has been very short. I'd recommend June and early July as the best time
3 For the amount of frustration most guys would have to face, I would honestly recommend to skip Serbia. Unless you want to save money and have other reasons to be based there, greener pastures are out there. Romania ,heck even Hungary are better places to go. Serbia like Ukraine has a lot of really really stunning girls, that doesn't mean you have access and quality is still overall hit or miss. Hungarian girls get and deserve a bad rap, however they will fuck you and the quality isn't overall that much better. I'd say 65 percent of girls in Serbia are between 5-7. 10 percent are really hot and the rest are below 5's. This is my personal experience but I'm not seeing the benefits. Girls play a shit ton of games, they can act masculine and bossy at times, they can be demanding. They tend to have aging skin, thunder thighs and big calves and broad shoulders. Overall its still top notch EE quality, but it's good to temper expectations.
Serbia is a good place though to save money and self development. Wouldn't live here for over a year though unless you have something else good non lizard related going for you. Also for all the hotties you get good reception from,some who might even fuck you if it wasn't for the village mentality, your ego and confidence will go up. I rather get eye fucked by hotties than actually fuck 5s and 6s regularly so it's not all bad, it's just not a nation I can recommend for game.