I did exactly what you're talking about.
I had an ash in the back yard that had to be climbed as there was not enough room to bring it down whole. I got quotes and because of the lack of access, the quotes were insane. The cheapest one for a single 50ish foot tree was $1800. I bought tall the gear to climb and remove the tree including the saw, for less than that.
I didn't pay for any training. Didn't pay a cent. Not that paying for training is a bad idea, I just didn't. YouTube has a numerous very smart and well trained people that are putting out very good information on how to climb and remove trees. Obviously there are plenty of idiots out there too..so you have to know enough to know which ones to follow and which ones to ignore. However, watching people fail at tree cutting is just as informative. Knowing what not to do is just as important as knowing what to do.
It took quite a bit of time. From the time I bought the gear, to the time that I actually took down the tree was about 16 months. I spent a TON of time climbing low and slow. Tying and re-tying knots. Setting up, tearing down, and re-setting up my gear. Doing it until I could essentially do it blindfolded. I probably climbed 50 trees without the saw before I attempted to remove the tree I had set out to remove.
I ended up taking down my tree without incident. After that I did numerous trees for family and friends. Then I started contract climbing for a couple local companies. That's the best. They cover all the hard parts, all I do is show up, climb and cut.
You can absolutely learn to climb and cut trees from YouTube. But you also have to be smart about it. Watching 20 minutes of YouTube and heading out with the saw is a recipe for disaster. Watching 400 hours of YouTube and doing a bunch of practice getting confident climbing before dismantling a tree. You're in much better shape. There's no guarantee. It's one of the most dangerous jobs in the world for a reason. There is nothing 'safe' about climbing trees with a chainsaw. Ever.
My 'advice' is to climb a bunch of trees without the intention of cutting them down. Know your gear inside out. Know how to get down quickly. Practice using a figure 8. Practice getting into good work positioning. Practice, practice, practice. Set up your gear, climb up 3 feet (literally 3 feet off the ground), climb down and take down your gear. Set up and climb to 6 feet, down and repeat, going up little by little. The repetition will teach you so much.
Honestly, climbing isn't that difficult. Cutting trees isn't that difficult. Most humans can learn to do it. Doing both at the same time...that takes a lot of forethought and understanding of both equipment and physics.
I would highly suggest climbing on a rope system. I would not climb only on spikes and a tree belt personally. Be tied in twice when cutting. Double/triple check your ropes when making cuts. If you're planning on rigging limbs make sure your rigging ropes aren't going to tighten around you or run on your safety ropes.
EDIT: DM me or @ me if you have any specific questions you want me to answer. I can give you more specifics about the gear I used or would recommend if that's something you're interested in.