I have a 2004 Corolla 2ltr D4D with 135k o the clock.
I would like to start maintaining her myself if it is doable.
What would I want to do to service her and how easy or hard are the tasks?
Right on! Go for it.
I concur - checking other threads is a great place to start.
In general, this is a pretty broad and open-ended question. It depends on a number of factors, like your current level of mechanical knowledge, etc... But if you want to approach it systematically, I would recommend starting with the easy stuff that is less likely to put your life in danger (e.g. changing wiper blades, topping off fluids, filling tires with air). Then, as you gain confidence, progress to tasks that are a little more advanced, in the sense that you will still be safe, but might damage the vehicle if done improperly (e.g. change oil/filter). The last step are items that could cause injury to you or others (e.g. brakes, suspension). Obviously you want to be careful and have safety in mind as a top priority. Also, web searches and youtube can be your friend. I would also recommend you find a knowledgeable friend and ask if you can work with that person to help train you in. Know your limits and don't advance to more complicated tasks until you are sure you can handle them.