Assuming we're discussing the 1.4ghz 2014 Mini in your signature.... the problem is, there's a real limit to how much you can "fix" it since the 4gb RAM is not upgradeable. The OWC SSD that you linked to should be about as fast as you can get for your Mini - which is good - but I'd just question spending another $219 on such a limited machine.
I also have the same Mini and used it as a media server for a number of years but replaced it with a faster 2014 Mini with more RAM. The old 1.4ghz model just sits in the closet now, it would be like spending "good money after bad" to upgrade it. You might at least consider selling yours for whatever you can get and applying that towards a used 2018 i3 Mini. That would be a much, much better machine.
I also have the same Mini and used it as a media server for a number of years but replaced it with a faster 2014 Mini with more RAM. The old 1.4ghz model just sits in the closet now, it would be like spending "good money after bad" to upgrade it. You might at least consider selling yours for whatever you can get and applying that towards a used 2018 i3 Mini. That would be a much, much better machine.