Advice needed - automate everything or only new/current projects
You can spend a lot of time, to automate something, which is done monthly and do routine staff manually on a daily basis - not the best way.

If you are time/resource limited, then find the bottleneck in the software delivery process and improve it, after that bottleneck is not a bottleneck anymore, find a new one and so on. So if you are spending 80% of your development time/resources on the new project, automate it first, and leave the old ones for the next step or vice a versa, depending where the main bottleneck is.