Redirect to a collection other than DefaultCollection
I used to have the same troubles as you. AFAIK, there is no such option to change/redirect the DefaultCollection.
Besides, after a period of research, I found that if I create a new Organization in one browser (like Google Chrome), whenever I log into Azure DevOps or access https://dev.azure.com, it always redirects me to https://dev.azure.com/NewOrg instead of https://dev.azure.com/DefaultOrg. However, when I access it in other browser (like Microsoft Edge), it still redirects the DefaultOrg.
So, I speculate this problem should not from Azure Devops but by browser. Then, to resolve this issue, I switch to the DefaultCollection, sign out current account, then re-sign in to the Azure Devops. It will redirects to the DefaultCollection.
When I log into Azure DevOps or access https://dev.azure.com next time, it redirects to the DefaultCollection. It works on my side, you can check if it works for you.
Hope this helps.