You can safely leave items in the “Product Improvement” catagory within System Services un-ticked if you wish - but the other items should ar this stage be left enabled.
Have you verified that each of the Apps that you wish to use are permitted to use Location Services?
As for comparing with a device running iOS 11.0/12.0, it is very possible that this will have some signficance - however, we are not seeing a vast number of complaints from other WiFi-only device owners. As to your final point, your original posts indicated that your wife’s device is running iOS 11, not iOS 12.
Other than suggesting that you update your wife’s iPad to iOS 12.1.4 - so as to compare performance of identical devices in the same physical location - assuming that you have verified correct settings and have force-restarted your iPad - you should perhaps entertain seeking help directly from Apple. You can either visit your nearest Apple Store as was suggested earlier in this conversation, or you can contact Apple Support using the Contact Support link at top-right of this page.
Best of luck resolving your issue. Do let us know how you get on.
I hope this helps you reach a satisfactory solution.
LP