Well the OP wants everything (non-cloud, flexible, for free). To put it straight: such a thing does not exist.
You always have to pay a price in terms of quality and work you need to put in.
But while non-cloud DIY solutions may not be fully on par with Echo and Alexa, it does not mean they aren’t good.
Check out Mycroft. I don’t have experience with the device they offer, but the software they use in there is available as Open Source, ready to work on a RPi3, and it comes with an openHAB skill.
Works with any microphone but I’d recommend the ReSpeaker 4-mic array which is ~25USD.