What about option #4 - Redirect your existing domain to the old domain? I bought an old domain that is 100% relevant to my current domain but currently has with very little content. It did have more content fours years ago. The old domain is 13 years old, pr=3, while my current is 7 months old pr=1 and a decent amount of content. The old domain I purchased was not expired though and I do not know if this makes a difference. What are the pros and cons of option #4? Am I correct to think that option #1 would result in no benefit from the old domain's age value and if so why is it not listed as a con, a MAJOR one. Its hard to believe that a 301 using option 1 would give my existing domain 13 years credit but I'll take it if it does.
If you bought the domain name, the typical procedure is to unlock the domain name, then either push it to your account at the same registrar (such as https://www.domainsherpa.com/how-to-push-a-domain-name-at-godaddy-com/) or send you the authorization code so you can transfer the domain name to a different registrar (https://www.domainsherpa.com/how-to-transfer-a-domain-name-to-another-registrar/).