The two obvious hurdles are crossing Biscay and getting down the Portuguese coast.
Our take on crossing Biscay in summer – any other time is out of the question for ordinary cruising folk – is that there are basically two alternating weather patterns: the Azores high reaching into the bay, giving northerlies inside the bay and calms on the direct Falmouth-Galicia route; and depressions swinging south in the Atlantic approaches, giving south-westerly gales on the direct route, but manageable westerlies further into the bay.
Our plan was to coast-hop to somewhere near Brest, then head for North Spain if a settled high arrived in Biscay, offering a guaranteed 4/5 days’ moderate northerlies, otherwise continue day-sailing down the French coast. If need be, we had time to sail all the way along the coast. Getting down the Portuguese coast requires having the Portuguese trades, which we thought meant we needed to be round Cape St Vincent before the end of September. The other problem is that the trades often blow uncomfortably strongly round the main capes, especially Islas Sisargas west of Corunna and Cape St Vincent, and swells from distant depressions can give horrible seas. The only way to cope with this is to have enough time in hand to wait a few days before each if necessary until the trades and/or swell ease.