First, we need to establish the flow rate requirements of the jets. I showed you documentation which stats that 20 GPM is required for each jet. You then responded that you still thought the jets required 10 GPM but did not provide any source for that information. However, the test you did confirms that the jets you have are 20 GPM jets. I estimate that with 8 jets, you have about 12 GPM/jet. Removing and capping 4 jets, results in a flow rate of 21 GPM/jet which to you felt strong.
So, if the jets are indeed 20 GPM, there is no way you are going to get that kind of flow rate from a residential pool pump on 2" plumbing. With those jets, the PB should have plumbed two loops with separate pumps. You would need to go with an extremely large industrial pump to get that kind of flow rate with a single loop but it would not be recommended to even try.
If the jets are 10 GPM, the pump you have now should be working fine but it is not. So I suspect that the jets are not 10 GPM but are 20 GPM. If you can actually find 10 GPM jets, you may be able to get the system to work properly.
BTW, if there is any doubt about the impeller performance, remove the motor from the wet end and inspect the impeller. It is fairly easy to do.