An interesting fact about circulation pumps - have you ever considered that because they’re circulating water in a closed loop, where it falls as much as it rises, the system is theoretically perfectly balanced. This means that there is very little pressure on the suction or discharge, and in fact any pressure required to circulate the water is only caused by friction losses in the pipe and fittings. Most other pumps are required to lift from a lower point to a higher point,
Have you checked your valves recently? It’s a problem we fairly regularly come across. When valves haven’t been checked or regularly opened and closed, they will eventually seize solid, making a simple job like disconnecting a pump a whole lot more time-consuming and expensive (often requiring the system to be drained down). Definitely worth making that part of your routine checks.