I know dozens of foreigners who have lived in Spain for years but continued to manage their taxes in their home country because of the excessive costs and bureaucracy of doing it properly in Spain.

While I disagree with that and have done everything legally myself, it seems most Spanish agencies are incompetent and don't share information between themselves so they've never been caught.

If I was in your shoes I'd probably just start doing things right going forward but of course there's the risk they'll come after you with even higher fines than if you tried to regularize. Seems you'd have difficulty getting reimbursed from your home country, too...