I'm not sure why you think WiX is losing support. Plenty of free support is provided here and on the wix-users mailing list. They also offer paid support options with SLAs and fixes. Other consultants such as myself provide free support and paid support also.
The WiXUI is a simple to use canned set of UI with limited override/transformation capabilities. You can do things like insert a dialog into a wizard loop sequence but doing so requires that you understand how the rows in the ControlEvent table area evaluated.
You can get an example here:
https://github.com/iswix-llc/iswix/blob/master/Application/IsWiXNewAddIn/MSISolutionTemplate/SetupProjectTemplate/UI.wxs
My uncommenting line 10 it causes it to consume this fragment:
https://github.com/iswix-llc/iswix/blob/master/Application/IsWiXNewAddIn/MSISolutionTemplate/SetupProjectTemplate/UI-CustomDialog.wxs
Lines 33-34 insert rows into the ControlEvent table that take priority over existing rows based on:
https://learn.microsoft.com/en-us/windows/desktop/msi/controlevent-table
If this is too limiting for your needs, you can intead not use the UI extension and simply take a copy of all of it's files from here and do anything you want with them:
https://github.com/wixtoolset/wix3/tree/develop/src/ext/UIExtension/wixlib
Finally if that doesn't meet your needs you can always create a custom bootstrapper application for Burn and use WPF/MVVM to do anything you could possibly want.
FWIW, I've been writing installers for 23 years and using MSI for 16 years. I'm the author for the FOSS project IsWiX which provides templates and designers to simplify automate most of this work. You can get an idea of how IsWiX works here:
https://github.com/iswix-llc/iswix-tutorials
1 hour free consultations are available for interested parties. Just shoot me an email or send me a LinkedIn invite.