Do NOT go to Spain too early and definately do NOT go to Spain on US 2. That is a game losing move.
Germany WILL blow those trannies out of the water on G3.
And she WILL retreat from EE, sending a few Inf from Germany to WE, landing her Ftrs in WE and massing her armour in WE or Germany.
She will be impervious to attacks on either Germany or WE on UK/US3.
Only modest UK 3 reinforcements can arrive in Spain.
On Rus 4 she can take EE for free, but it matters not.
On Ger 4 she will have a MASSIVE armour & air heavy attack on Spain....but she won't take Spain....just kill 90% of your forces and then retreat.
Or she will throw everything at EE and blow out the Russian, evac WE and then turn around and kill you in WE next turn.
You will have no US 3 reinforcements, since you'll have to rebuild the trannies.
You will have no US 4 reinforcements, since you have to build then men as well
Basically, this leaves the US without fleet AND without follow-on forces in the US...she needs to build BOTH again.
UK 3 forces in Spain are just more dead meat and make the strafe more juicy.
UK 3 forces in Finland or Karelia are useless and cannot help defend the Russkie if she moves to EE on R4 from getting killed on G4.
The Russian/German front is not as scary as you think. The Russian supply line is extended by 1, while the German supply line is reduced by 1. Unless disaster strikes, Germany can deal with one front then turn to the other, without serious risk.
The only real risk to Germany is if they accidentally take Spain
So, you lose the tranny fleet and most of the forces, while Germany will take very minimal casualties.
Establish the pipeline first.
Get enough combat power in Karelia that it cannot be threatened, and it poses a real threat if EE is evac'ed.
Then land in Spain and squeeze...as noted usually UK first then US with Ftr support. A one turn double load of 16 troops or so with air makes a hit on Spain unlikely. Especially if the armour MUST defend EE or Germany and can't afford to move. This is a bit later when enough forces are in play on the Allied side.