What are you using to break the bead ?
Spoons are for light tires or last resort for anything over a 14". You need a decent "platform" that will have a bead breaker and a center post for leverage. HF sells one and so does Northern Tool both sell them for not a lot of money.
Manual Tire Changer Comparison - Harbor Freight 69686 vs. Northern Tool 13474
The key for mounting and demounting
(after the bead is "broken") is the bar and the leverage you get with the center post.
Now the ULTIMATE manual tire changer is the
No Mar Classic Motorcycle Tire Changer. Yes, it is aim at changing motorcycle tires, but
watch this video that show a guy changing a tire from a Hummer (looks bigger than 20" to me). One spec sheet said the "Classic" model would handle tires up to 22.6" !
The real key to the No Mar system is the bar. The "never scratch" tip have a lifetime warranty. Some guys will use a cheap stand (HF or Northern Tool) and buy the No Mar bar.
The real key to changing any tire is keeping the opposite side of the bead you are working on in the drop center ! I can not emphasize this enough. No Mar makes several tools to do this, but a couple of hardwood
(not pine) will work well also. This is usually a problem when demounting the first bead and when mounting the second bead.
The second key is lubrication. While some people swear by
RuGlyde, No Mar sells a
tire lube paste that works well
(once you know where to apply it).
The sad thing is this "paste" is basically Murphy's Oil Soap Household Cleaner paste which is discontinued. The stuff was great and cheap.
If I haven't convinced you to buy at least the Northern Tool manual tire changer, you need to check into
Ken-tools. There stuff is really aim at HD truck and larger tires. If they don't have it, nobody does and much of it is sold on Amazon !