I don't use Spine, so can't give any helpful tips regarding that. But I do suggest taking a look at Blender deeper, it's a powerful free tool that can do a lot of things, granted you take the time to learn it. With that being said, there are a lot of things that go into the animation pipeline, alot of what I'm going are from the use of Blender. Rigging, weight painting, pose controlers, drivers, NLA tuning, animation graph editing, constraints.
Start with rigging first, understand how it's setup, the relationships between bones as things can get complicated quickly here. Fine tune it with other bones that don't deform but control what other bones do. Create constraints, mainipulators, visual icons that help you move an entire section, for example, curle the hands or move both eyes.
Great tutorial on rigging
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Next would be weight painting. In the simplest form, it's how much influence the bones will have on parts of the object. If done incorrectly, you'll have strange results when you start posing. This could take some time, since fine tuning is needed here if you want to object to move correctly. Just do a Blender weight painting and any tutorials will get you started.
Posing. Once you finished the top two, learn how to use the controls you created during the rigging phase. You can create drivers to further make the posing easier. Though the topic on drivers are a lot more complicated, advance and takes some time to setup. But you can do all sorts of cool stuff and make the workflow a lot easier down the road. For example, during penetration, instead of posing the opening of the vagina, you can use drivers to tell the vagina to open when the penis is within a certain distant, and close when the penis is out.
Quick explantion to get you an idea of what drivers are.
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Animation. Once you're comfortable with posing your objects, move onto animations. There is a lot here, so I'll go over what I believe are key topics you should learn. The first is the fundamentals of animations. This isn't specific to one software but as a concept. Build a foundation on the principles of animations before jumping into it will make your animations look at lot better.
Second, learn the tools you are using. In this example Blender. There are great tools that you can utilize to tweek and fine tune your animations once you have keyed the poses. NLA tool. Best explained by this video,
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Third, the Animation Graph Editor. This is where you fine tune the flow of your keyframes. A great and quick explantion video of what the graph editor is.
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With all that said there are lots of topics, advance techniques and workflows out there. Start with these as a foundation and build upon them as you explore and experiment along the way. I'm by no means good at animations nor have I done enough myself, but these are what I have come across as I was learning what goes into the animation pipeline.
There's also an alternative to Blender, or rather a fork of Blender called BforArtist. It has everything that Blender has, but has been revamp for artist. Link to their page below.
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