I am a Norwegian. Norway is a part of EFTA, but not the EU.
I am currently pro EU, but also fairly pro socialism, or at the very least pro social democracy.
I want Norway to have the security guarantees the EU offers, I want the same currency, I want Norway to be able to participate in changing EU, not just accepting something Brussels have decided without our participation (EFTA). And several other reasons.
We can currently veto implementing some decisions, but not too many.
If Norway joins the EU will it make it harder for Norway to transition to socialism/maintain social democracy?
Norway will not be able to transition to Socialism through reformism. Social Democracy is incapable of swapping the balance of power from Capitalists to Workers. Norway depends on Imperialism, as does the rest of the Global North. As this weakens over time, so too will social services. The only path to Socialism for Norway, to have the public sector as the principle aspect of the economy and the Working Class in control, is revolution.
The EU is Imperialist, but thoroughly dependent on the US, and as such are tied to the fate of the US.