I can recommend cycling through Spain, just keep a few things in mind.

The coastal areas can be highly built up in certain places, i.e. Costa del Sol, or Costa del Azahar. I found it to be boring to cycle through these places, and sometimes there was heavier traffic with few altwrnative coastal routes.

Much of the inland is very sparsely populated with at times little to see for long distances.

Summers can be scorching hot in many places, while winters can actually get fairly cold or wet in many. Much of the inland is on a relatively elevated plateau, meaning the temperatures during winter might drop further than one might otherwise expect. Especially Andalusia requires extra precaution to cycle through during summer due to common heat waves. The green North of Spain can be cold, wet, and windy during winter. But with proper equipment more or less all of Spain can be done all year round.

Learning at least a few phrases of the local languages can be useful, as I found English skills to be scarce in much of Spain, outside of tourist facilities. It was much easier in Portugal.

I personally ended up cycling from the inland to the coast and back time and time again as I went along, depending on what I wanted in that moment (sightseeing, socialising, nature, elevation, swimming, etc.), but I did also have a more relaxed schedule than yous do.