Eli-marie, I can only really repeat things that have already been said - Spain is a safe country - the
Camino Frances will not be crowded in April - the Norte will be very quiet.
I will add some bits though: I walked the Frances in March -May two years ago, arriving in Logrono on Good Friday as the city prepared for the great processions. The municipal albergue was "completo" but i found a bed, no problem, in the paroquial Albergue Santiago and had one of the high-light nights of my pilgrimage. Never had a problem finding a bed the entire trip. Sometimes I almost had Albergues to myself. I had set out seeking some solitude and some time to think and had all that I needed, I also encountered wonderful company. I have some experience of stretches of the Norte and while much of it is rural you will also pass through some major cities, ports and resorts and the pilgrim infrastructure is much sparser than along the Frances. You may also find that some of the Albergues and other accommodations are not open so early in the "season".
All that said there are bound to be Norte enthusiasts among members who will be eloquent of its charms. Personally I find Santander a wonderful city, Santillana del Mar exquisite, Llanes has my favourite sideria on the whole coast and Poo is worth visiting if only to take the inevitable photo of the boundary sign. (The Camino is now routed so that you do not need to await the falling tide at Poo to cross the estuary.) At Ribadesella you have the chance to view per-historic cave paintings. At Gijon you can try the museum of the bag-pipe. And I refer only to a 150k or so of its delights.
There are some excellent threads on this forum on personal safety, and specifically for lone female pilgrims, that will provide sound advice but also re-assurance I am sure.
Buen camino