Hey Ed,
Do you want to avoid the "cloud" because you have poor or intermittent
net access? Or just because you don't want to want to trade the
convenience for your data?
If the former, I can highly recommend using Nextcloud, which can be "in
the cloud", but on a server *you* control. You can also install it on a
spare computer lying around your shop. It has a "Contacts" plugin that
can sync with a smartphone, but you don't have to sync it if you don't
want to (requires a DAV (distributed authoring and versioning) app such
as "DAVdroid" on your phone). My experience with Nextcloud is 98%
positive: the only 2% negative is probably from the old Owncloud days
when upgrades didn't go as smoothly as they do now. I've upgraded
(incrementally) from Nextcloud 9 through the current Nextcloud 15 with
Zero hiccups.
(Not to hijack this thread, but I'm currently struggling to get the
Collabora shared document editing working on my nextcloud instance. If
anyone has clues to get it working I'd be very thankful!)
I have no experience with POS apps.