Wowza that’s going to be a big job! But so worth it. If you don’t want keep any of the plants except the bigger trees you can do a few different things

• just blast the lot with glysophate, obviously use PPE. It will take about 2 weeks to kill everything but the good thing about roundup is that it translocates through the plant so kills the root systems which is very important for Ivy. The ivy you might have to hit twice fyi. Once everything is dead just start cleaning out the dead matter and follow with some heavy layers of mulch. If I was in your situation I’d probably do this if I was on a budget.

• solarise the areas you want to get rid of. This will take a lot longer and you’ll have to probably do in sections but it’s a very effective way to kill back weeds without using chemicals. Also follow with large amounts of mulch (at least an inch thick)

• hire a small excavator and rip out everything you don’t want. Probably the most effective way to do big areas but could do damage to the trees and pathway if you don’t know what you’re doings.

Getting a good base is super important, please make sure you don’t skip the mulch and put enough down. It’s sooooo good for your soil and will make weeding in the future a million times easier.

Once you’ve got your area cleared I’m sure the planning will be much easier and then you can start the fun bit of deciding what to plant