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I'd prioritise 2, 3, 5 in that order. I don't know APT but if it's like Backyard EOS it will give you good feedback about focus. I suspect you are concerned about focus in your subs when in truth it's the guiding that's causing distorted stars. See how you go with the focusing with APT before throwing in the mask or the motorised focuser.
Just my 2c
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