For the discontinued products, is there a new model/version of that product or is there a similar product that isn’t discontinued? Or is there helpful guides or information about the product that people would still be searching for?
What is the search volume for the products that are discontinued?
If you have similar or newer products and there is decent volume still I would keep the URL active but change the content on the page to say that this product is no longer available with a CTA pointing them to the newer or similar product. That way you will appear in the SERPs for multiple intents – service/support of the old product and transactional for those that want to purchase.
I tried to find an example at [REI.com](https://REI.com) because they do this, and it was something I did at my last job and we saw decent conversion on new devices from traffic that landed on the discontinued device URL.
In terms of how long do you keep them, I would watch the traffic and volume. Once it gets low and/or you are no longer driving traffic/conversion then I would redirect to the relevant category page – if possible filtered to that brand.
And to your last question, no there is no harm in having redirects – though after time if you discontinue the full category then you have to go back and update all of those redirects to a new destination.