What is the best way to hold back dirt on a slope of about 60 degrees?

Perhaps a retaining wall or a graded landscaping?

I would build it up like a terraced landscape. This would prevent erosion and would also give you a nice look.

Railroad ties, or landscape blocks; labor intensive;

Consider a look around Pinterest or Houzz for visuals that might interest you with a landscaping on an incline search. Thats where I collect lots of ideas from.

If you dont want to mow grass -grown covers could be an option. They tend to hold the soil some —to prevent erosion.

Think too of looking around your large commercially landscaped areas from supermarkets, car dealers, restaurants etc , often they are off main highways elevated having inclines with substantial tree and shrub plantings or rocky walls. Certainly you can consider some of these concepts too.

Remember its step by step, nothing in landscaping is overnight it may seem slow at first —-but 5 years of tree growth can be quite remarkable!

See leylands below.

Best wishes in your landscaping adventures!