I also had to throttle my ground floor overall to ensure sufficient flow to the upper floor. This is normal. On the ground floor, I have only two out of eight circuits almost fully open; the others are throttled from slightly to strongly. On the upper floor, most circuits are almost fully open.
As an example: our ground floor hallway is large, has an 80m circuit, and is fully open because it needs to heat adjacent rooms, the front door, and the stairs. The upper floor hallway is small, also about an 80m circuit length with tight pipe layout, no windows, and it borders warm rooms (bathroom, children's room, office). The upper floor hallway is quite strongly throttled. However, the hallways on the upper and ground floors end up with similar return temperatures. Hence the tip about the IR thermometer—this gives a good sense of how much heat a circuit or room needs and can absorb. The room temperatures in the hallway are of course irrelevant; the goal here is to avoid short circuits while still delivering a lot of heat through each circuit into the house.
We also often have relatively low flow; our heat pump regulates this dynamically (regularly only 600 l/h for a total of 17 circuits). Nevertheless, the temperatures work out because the temperature spread is simply somewhat larger or the flow temperature is higher. High flow lowers the flow temperature; if you can increase it a bit, try that.