I dont think senku and kohaku would be the best choice for senku in the end. If anyone I think Gen Asagiri would be a Better romantic partner for senku in many ways. Gen isnt very secure he actually seems a bit insecure at times, when he was beat in poker he took it quite hard and acted impulsively and very dangerously to try and make himself feel better, i think senkus blunt honesty and straight forwardness is refreshing for him because he doesnt have to second guess if hes wanted. Around gen senku can be more like a kid, we see the scene of them together at the start of seasomn2 where they are in a hut together, senku planning and gen making wire, gen is enthusiastically singing senkus praises and telling the story of senkus depertification (which I assume senku told gen) to the kids and senku makes exasperated comments at gen for slacking but the fact that senku is there is interesting, gen is making wire with the kids and supervising, senku is doing his own thing but hes not in the observatory or in his lab, hes watching the kids with gen. Its also stated that senku plays with them occasionally in the manga awh. Senku can be a bit ridiculous with gen and gen will return the energy, I think gen can tell how stressed senku must be sometimes, actually gen says it blatantly, that he hopes Lillian's singing helps relieve the guilt senku feels for dragging everyone into the stone wars, forcing senku to acknowledge it. He also knows senku is not one to be coddled though