Is it ok to 301 redirect product-category pages to custom pages?

You typically won’t experience too much of an issue if you’ve correctly 301 redirected the URL. However, it’s better to customize an existing page instead of trying to replace it – and as you said, end up with incorrect breadcrumbs, which can make for a confusing user experience.

If you create a child theme, you can create a product archive template for just the one product category. You can then either just place your desired content directly in the template, or if you really want an Editor experience, you could create a Page (being sure to 301 redirect it to the product category, and being sure it never gets listed in any sitemaps) and within your product archive template, query for that Page’s contents so you can edit content within wp-admin.