I sort-of do both, office job and forestry / log work. (Not a qualified tree surgeon, but send about 20hrs per week doing tree & log work.) Run a side business selling logs, clearing pre-felled trees for farmers and private land owners, alongside being a buyer for one of the big online cycling retailers.
My advice is, if you’re going for it, you need to be 100% committed to your decision. Be aware that you’ll have days stood freezing cold in the middle of nowhere, putting yourself at huge physical risk for little reward. You will more than likely get injured and be out for weeks. Plan for it, I don’t know a tree surgeon who hasn’t.
Also, if you’re going self employed, be aware it’s not just solid money rolling in. Good kit can cost well over a year’s income. I started as a side earner with limited kit, but quickly discovered you need to be at it for a few years to really get the benefit.
The good days are great, the bad days are **** awful.