Frank, you do what you are comfortable with. When acquiring cash, I cannot agree with carrying €50 notes. If that is all you have left and you walk into a donativo or even a municipal albergue, there may be a wait to get the change needed to complete admission. Take every opportunity to use €20 notes as most purchases will require less than that. Yes, change cand add weight but not too much.
Some will use ATM's but for most, not all, there are fees that go with each withdrawl.
Use ATM's at OPEN Banks ONLY. If a card gets eaten, you need to do all possible to retrieve it immediately. Also, if €50 notes are received, they can be changed in the bank with little issue.
Now, for me, I am budgeting €40 per day for 40 days. If witdrawing €300 at a time, (ATM Max withdrawl allowed per day when exchange is involved), only repeating once my purse holds €100 or less, that means a withdrawl every 5 days or 8 withdrawls over the pilgrimage. Cost will be FEES X 8 = Cost of Withdrawls (Consult your bank for your fees). There will also be a percentage cost to exchange from native currency to Euros. The only way to get past this is to have an account containing Euros. Not all banks allow foreign currency accounts.
Now, I do try to provide some bonus information and I will try not to disappoint here.
The need for an emergency fund is rarely consideredor, from my experience, has been discussed here. It is wise to consider medical insurance as well. However, though available, med insurance is not usually so inexpensive for long duration vacations. We know it is not a vacation but that is the industry's terminlogy. And explaining it to a travel insurance agent, for me, has ended with no sale, ie, been refused coverage. Purchasing such coverage should follow the needs of your personal situation.
Hope this helps.