Thank you all for the suggestions. It seems that using gifts is key. I've restarted the campaign as france and allied with spain, denmark, venice and the pope as fast as possible. I also eventually allied with scotland. Those 5 are the ones I keep giving gifts to, in order to keep relations at amiable or better.

I took caen as fast as I could, after securing reims and then immediately accepted a ceasfire with the english. My diplomat parked next to rome sent a gift to the pope the moment the ceasfire was signed, so my relations with the pope were back to good/very good.

Taking caen and then bruges was a significant boost to the economy. The danes took antwerp but I am allied to them so they did not continue towards my town (although they seem to be the ones requiring the most money to stay amiable). I then spent several turns in peace, building up armies and most importantly, economic buildings in all settlements.

Interestingly enough, neither the HRE nor Milan or Portugal have attacked me, probably because of the large number of alliances I have plus the fact that I have a strong army presence (that I can finally afford). Just recently, Milan got excommunicated, so I grasped the opportunity to call a crusade towards Milan (I wanted Genoa but the pope refused that one!). Having taken Milan, I am expecting to be done with the annoying milanese in the next couple of turns.

This has definately been a rather more entertaining campaign compared to my first 3 attempts :)