With that level of criticality it's probably best and easier to stick with something that works for you
However I did notice now we have more control of the cloud backups it's possible to manage several or more device images ie I had S7, S8 and S9 backups that presumable I could restore along with 2 different secure folder back-up which may offer additional assistance for you in addition to physically keeping the old phone. This was on top of the back-ups I had on my laptop.
Given your requirements I certainly I would be cautious and look in to it more and even do some product testing but from my limited experience as a normal user on face value it seems possible to achieve the same and more should you swap platforms
Best of luck
Yes, that level of importance has been primary in keeping me with the iPhone since I moved back to iPhone in 2012 / 2013 (approximately). I did lose one message during my transition back, that was unfortunate. I was setting up the iPhone, and had a message come through, and then had to restore to factory due to a problem that couldn't be resolved. Was like a matter of minutes between turning it on, having the message come in, the phone crashed and died, and I had to do a factory refresh to bring it back. And, the loss of that single message cost me 3 years in court. I had to pull other people in, and it was a mess. That one message could have made the difference of it just being thrown out of court. I lost big time in that one, but while remaining free, it put restrictions on my freedom for a long time.
It sucks having people in your past that do things, then try to claim it was you. My phone and computers hold the archives of the truth. I've cleared my name, but still have people trying to drag it through the mud. And, have another person who tried having me jailed only a few months back, but the text messages contradict their own story. So, I remained free. But, they keep trying. The statute of limitations for the offense they allege is 10 years. So, I can't lose messages for 10 years from now, or I'm basically screwed. The police know I have the messages, and they held a grand jury hearing to decide whether to arrest me, and based on evidence determined they couldn't arrest me. But, the person has continued to file allegations, and also dragged people in town into it. So, when I get approached by the law, or other people that have been sent after me, I pull out the phone and show them what is in writing. It clears me fast. Some people are vicious when they want something they can't have, they try to make you pay. But, enough of that lol...
I have stayed with the iPhone due to the ability to archive everything and put it back as it was. And, the same with the calendars and contacts automatically syncing with my calendars and contacts on the computer from my phone. I can always say what I was doing on any day that I'm asked about going back to 2009. Which sadly is important. That level of microscope is annoying.
But, I've also been frustrated when I could get a free Android phone vs. a $700 to $1000 iPhone. The thing I like about Androids is the ability to put a flash card into it, of whatever storage amount I desire, and handling media / files / music / and documents as actual files that I can move into a traditional directory / folder format. And, I can pull that content or add content to / from that card by simply plugging the card right into my computer. That is the simplicity I miss. Or complexity perhaps depending on your viewpoint.
That feature actually saved my butt in a court room at one point prior to my switch to iPhones. I was asked to produce something in the court room, and could pull my memory card out, and they could play it on the court's computer / audio system. And, the audio kept me out of jail. Stupid fools, yes I record if you come near me lol...
But, it had to be played on the court's system, and be something that was a file which could be duplicated and handed in file format to the court / judge, the accuser's attorney for analysis to have the ability to try and prove it had been altered, and to my attorney to have as well.
That's a level of functionality missing in the iPhone. Not everything is treated as a file. And, not everything can be removed from the phone / transferred as a file to another device without losing it's metadata of creation date. If I can move a file from the memory card using a file manager, it's creation / modification metadata remains intact. If you send a file from the phone to another device, it now has a new creation date when it arrives on the next device. That presents the impression that I just made the file.
So, I really prefer the way Android treats media and files, and has removable memory cards that I can change easily and store. The iPhone forces me to pay a premium for greater storage capacity, and I can't add to it, remove it, or directly manage the files from another device as easily as I could with Android.
But, I could never find a consistent backup / restore solution that treated all Android devices the same. Such as if my phone died, and I purchased a different brand or model Android, could I make it look exactly like the phone that died.
From what you're describing, it sounds like perhaps they've begun to add the level of functionality that I would require. I might have to look into that. If it is there, and appears that it will remain there, then that might increase my future options. I'd then only have to figure out how to migrate text messages and such over to the new phone if it was not an iPhone I moved to.
But, it is encouraging to hear that they've begun to implement those types of features.
