We've used landscape professionals before with mixed results. But honestly, as a long time landlord, I would not spend a lot of time and effort on someone else's property. Even though your intention is to stay there long term, you cannot predict what the landlord will do. Worse case scenario you make improvements on the yard, spend your own money and put a lot of work into it yourself and then the landlord decides the property can bring in higher rent. Or he might decide it's sell-worthy- based on YOUR improvements. Weirder things have happened.
In your shoes, I'd find a student in a landscaping program if they would give their opinion. Then I'd clean up the yard and ask the landlord if he will buy some shrubs and perennials for you to plant on his property. That seems like a good compromise.
" I object to violence because when it appears to do good, the good is only temporary; the evil it does is permanent." Mahatma Gandhi
"This is the ultimate weakness of violence: It multiplies evil and violence in the universe. It doesn't solve any problems." Martin Luther King, Jr.