I have sympathy with rejecting the IHRA definition of antisemitism. The problem as I researched this issue is that verbal attacks on the state of Israel lead to physical attacks on Jews in the UK. I also deplore the wilful ignoring of UN resolutions on illegal Israeli settlements, which makes a two-state solution more difficult to achieve. I think the reality now is that there will not be a Palestinian state as initially envisaged by the UN in 1947.
The IHRA definition should, as you say, be rejected. It is antisemitic in itself according to its own definitions. By conflating Israel and being Jewish.
Whilst I have no reason to doubt that physical attacks on Jews are real and based only on their being Jews which anyone condemns, I think that it is a stretch to link "verbal attacks" on Israel to physical assaults on Jewish people.
The illegal settlements, and the "immigrants" they attract, are the biggest problem, as you allude. This cannot be seen realistically as anything but deliberately preventing a solution as envisaged by the UN. The side sabotaging this is undeniably Israel.
The 1947 solution is, as you say, further away than ever.
If Starmer were to confirm Palestine's right to exist, what do you think would happen?
He would be smeared and hounded out in no time flat.
He could make any pro Israel comment and there would be no such result.
It is that influence on UK politics and Starmer's Labour Party that is cancerous.