Yosemite, iOS 12 and iTunes 12 compatability

I keep my iMac on Yosemite 10.10.5 because i have lots of non-subscription software that won't run on later OSs e.g. Adobe Creative 5.5 Design Premium.

I have updated my iPhone iOS from time to time (now on 12) but upon connecting it by usb to my iMac for the first time in a while, it says I will need to update my iMac OS to 10.11.4 for it to work with iTunes.

I don't want to do this but would like access to my iTunes library and perform syncs and back ups.

System Inofrmation app shows that iTunes went to 12.7.1 on 2 Nov 2107, to 12.7.2 on 8 Dec 2017 and to 12.8 on 15 Jul 2018.

iOS 11 seems to have arrived in Sep 2017 and 12 in Sep this year although I'm not sure when I installed them.

Is it the iTunes (12.8.0 150) or the iOS that's preventing the connection? I see that unknown to me the last back up was in February this year.

Any help appreciated!