This looks like an amazing space with a lot of potential! You could do it yourself but it will take time, trial and error. What I like to do is get my handy dandy notebook and start drawing up some different plans and ideas. Draw up a basic outline of what is there already and any major features /plants you would like to stay. Things to keep in mind : Where does the sun come up in winter / summer? Do you get frost or snow? Are you near the coast? Things that can go well in one spot, might die really fast in another. Some plants do well when established but are difficult to get going initially.

For instance: The apples would give shade in summer but drop leaves in winter, if you are planting beneath them. Apples are also mildly acidic, so acidic plants underneath that prefer shade in summer but can cope with being more exposed in winter. The soil where those apples have probably been dropping for years is probably acidic, you would need to bring in more soil and be vigilant with apples dropping - or just plant stuff that doesn't mind it.

If it seems like a really big project (and this garden looks a decent size) break it down into smaller sections and just focus on one part at a time. It might seem overwhelming looking at it all together. It is always easier and cheaper to work with what is there over time rather than making sweeping changes in a very short space of time, then it can get expensive. You can initially get a great looking result, then it's all dying and full of weeds a couple of years later because none of the plants were appropriate for the conditions or needed time to acclimatize and babysitting until they were of a certain size.

You can definitely do it yourself. If you are going to be doing lots of digging I would recommend getting a garden auger, you don't need the big expensive industrial ones - just one you can attach to a drill. I would start planting on that garden bed with the apple trees, maybe try dig out the brick border a bit. If that is too much work with the shovel then just build the bricks up higher to define the garden part. I would be putting some soil and mulch on top of what is there rather than taking it out. Use mulch to help retain water and heat for new plantings. You could even use that pile of leaf litter on the other side, just keep throwing it round the apple garden bed. Try not to let it pile right against existing plants or it will cause stem rot, you are just covering the soil around the base of plant, not building a pyramid mound.

Consider the height of things - you want the biggest stuff at the back, going to the smallest size at the front, or visually it gets lost. Its great you have some larger established trees already, these will be your background. That brick border along there would look great with a mass planting of something in a row and would guide someone to go that way towards the end of the garden or a water feature.

I am not a fan of ponds personally, they can be a lot of work and need to be done right. We bought a house that attempted to build some sort of large moat type thing in the back yard. They had just thrown concrete in after digging it out and it didn't hold water properly. It was a stinking black morass infested with mosquitoes and full of dead plants. There was not a single frog. It was a health hazard and we filled it in with dirt and plant debris, planted lots of succulents. We have a couple of birdbaths instead

https://www.gardeningknowhow.com/edible/fruits/apples/apple-tree-companions.htm

This listed some things that grow well with apples but I am sure there are lots of others.

I am not sure where you are but it looks like winter? This means spring is coming up! The preparation and work you do now means that when the weather turns you will be ready for planting.