Of your different categories, which ones tend to get the most inquiries? Which tend to get zero interest? How long have you held those which have had no inquiries? What are the renewals on any CCTLD or new TLDs? Expensive renewals need to be given more serious consideration for deletion - particularly if they are not getting inquiries. Do you plan to develop any in the future (even if you outsource the project)?
1. Does the domain has liquid value? (Worth something on the after market,4L .com Great Brandable names) if so, renew or flip it..
2. Will a company want that name? if not, drop it..
3. Is the name is similar to a TM? If so, drop it..
4. Is the name makes sense? If not, drop it..
5. Proper spelling? If not.. drop it unless it is intentionally misspelled like Burnerr and Qrumb
6. Does similar domains sells? Look at namebio.com. If so renew it..
7. Have you received any inquiry for this name? If yes, renew it..
8. Would you use that name if you were a startup? If not, why someone else would want it? Drop it..
9. Is the name getting Organic (not robot) traffic if parked? If so renew it..
10. Ask the NamePros community for feedbacks on this particular name if needed