There's several things I've done to mod the game to sort this problem out. Do a search for info on how to edit the startpos.txt files

Scrapping some of the land bridges and province borders can make things easier for weaker factions. If you scrap the Wessex/Flanders land bridge, France cannot invade England without building up a navy. I also scrapped the Saxony/Denmark border to stop the Danes from conquering northern Germany within 20 turns. Also, try scrapping the Constantinople/Asia Minor border so the Turks need to either build up a strong navy or conquer the steppes, eastern Europe and the Balkans to get at Constantinople.

If you change the AI behaviour of all the catholic factions to Catholic_defensive, they will concentrate more on building up their defences and economy than launching half-assed attacks on their neighbours.