I actually read quite a few of your posts; e.g. [OH3] Semantic Model setup via tags in configuration Items files
Yes, had a look at these, but the 'big picture is what I am missing.
It favours the UI, but the case for ‘pro’ is weak.
Do I need the model?
If so, does it need to be done first?
Assume I modify my items files suitable for import, can I later bring these into the UI?
Is a mixed mode possible? Leave existing stuff in text files and add new stuff in the UI (if this is ‘strongly recommended’ approach).
What must be configure in the UI? Things, groups?
What can stay in text files?
Well, guys, don’t get bonkers over what I meant… 
I moved from v1 to v2 and may have or not used specific v2 functionality. Can’t really tell; too long ago.
These are the only ‘things’ I have:
// interval = minutes for astro
Thing astro:sun:home [geolocation="-27.164855,152.701568,200", interval=60]
Thing astro:moon:home [geolocation="-27.164855,152.701568,200", interval=60]
Thing astro:sun:minus90 [geolocation="-27.164855,152.701568", interval=60]
{
Channels:
Type start : set#start [
offset=-90
]
}
and I apologise for not having included some system info:
* Platform information:
* Hardware: Raspberry Pi 3 Model B Rev 1.2_
* Host: rpi3ohv2 Kernel: 5.4.51-v7+ armv7l bits: 32 Console: tty 2
* Distro: Raspbian GNU/Linux 10 (buster)
* OpenJDK Runtime Environment (Zulu 8.31.1.122-linux_aarch32hf) (build 1.8.0_181-b122)
* Version: 2.5.11 (Build) (apt-get), text-based config
* binding = astro, exec, logreader, network, ntp, systeminfo, fritzboxtr0641, expire1, mqtt1, weather1
* ui = paper, basic, classic, restdocs
* persistence = rrd4j, mapdb
* action = mail, mqtt
* transformation = map, javascript, xslt, scale, jsonpath
There are a few v1 bindings in there; which I am aware need some modifications, some of which I have made and commented out.
This is a representation of my item file style:
String spm_MAC "MAC address [%s]" <network> (gSpProModbus) {mqtt="<[mymosquitto:ArgyleCourt/Shed/Modbus/MAC:state:default]"}
String spm_IP "IP address [%s]" <network> (gSpProModbus) {mqtt="<[mymosquitto:ArgyleCourt/Shed/Modbus/IP:state:default]"}
String spm_Version "Modbus controller firmware [%s]" <settings> (gSpProModbus) {mqtt="<[mymosquitto:ArgyleCourt/Shed/Modbus/Version:state:default]"}
String spm_Name "System [%s]" <settings> (gSpProModbus) {mqtt="<[mymosquitto:ArgyleCourt/Shed/Modbus/Notification:state:JS(splitMqtt0.js)]"}
Rules are all DSL based.
I am on DarkSky which I replace with OpenWeather.
Want to use mosquitto as MQTT broker.
It seems that file-based and UI-based items can coexist?!