curious iPhone user here

[Usedname]

The non-pro Pinephone is definitely usable, but the specs are not robust.

Have you considered getting a Pro at some future point, and in the meantime get your feet wet on a converted android phone running mainline linux (Postmarket - OnePlus 6/T), Droidian or Manjaro Halium ( Pixel 3a/XL), or Ubuntu Touch (OnePlus 5/T, OnePlus 6/T, Pixel 3a/XL)? Sailfish and the Volla is also an option.

OnePlus 6/T seems to be quite decent, and there is also a lot of positive feedback about postmarketOS in comparison to Ubuntu Touch. So although I wasn’t thinking about it at first as I also noticed all the other gadgets from Pine64 that might become interesting for me too (I like getting everything from one place ), I might start with it to learn the basics in a stable environment. I guess this would be wiser than having to do so while also busy troubleshooting?

editing Mindmaps, browsing internet, Apple pay, listening to music, and basics like mail, calendar

can all be done. Battery life is not super but addt the keyboard and you have a long life nano laptop. ApplePay may not exist. Pinephones are slower than contemporary mobile phones. They are a platform for using mainline linux. You ccan install Nextcloud if you don't like some native apps. ( as good as walled garden offerings).
I use manjaro lasma mobile which is quite much a work in progress: There is a Nextcloud connectivity, but not for the calendar. Kalendar however will sync fine, but it's not the best in terms of useability.

Since the on-screen keyboard seems to be a powerdrain, and I often use dictation anyway, will speech recognition work on it? Mycroft seems to be in these contexts the counterpart to Siri, no?