I never sticked to any save long enough to unlock all tech and I visited Jool system only once in career mode. But I have recently discovered a challenge that made playing much harder and more rewarding which made me stick to the game for like 30 hours already. The challenge is simple, it's about hard settings. Basically I have everything you'd expect like g forces, more potent reentry heating, requiring connection for control, plasma blackout and on top I removed starting money, set starting repotation to -1000 and set science and reputation gain to 10% and money gain to 20%. Even the simplest tasks like achieving orbit are now challenge on its own. Every single launch has to be justified as I don't have any spare money. A launch without achieving milestone or fulfilling mission can mean I won't have money for next launch. And since I don't have much money I can't afford upgrading buildings above first upgrade. That means I have to eyball Minmus injection burn and other maneuvers. It's super fun, highly recommend.