The problems you mention that you haven't solved are solvable. Are you looking for advice? I admit, I don't quite understand the reason for the opening post.
1. Uploading my Signature in Preview and being able to sign and edit PDF documents on any of my Apple Devices.
2. Screen rendering quality, on Mac everything seems to be sharper and more crisp than Linux on the same device.
3. Keynote. I so want a cross-platform replacement that is nearly as good.
4. iMessage integration with phone. I can send sms/iMessages from my Mac.
But I am planning to switch anyways. As all these companies move to a "service model", I feel like I am only leasing my software, and tools that make the hardware work. Furthermore, it feels impossible to get under the hood anymore.
Would love any advice or recommendations for the above.
For 4. There is kde connect (you don't need to run kde to have it). Works only with android though afaik.
Those were about the only reason I could tolerate a standard Centos or RHEL desktop, but I'd heard the maintainer 'disappeared' and nobody has picked them back up.
And it’s the best basic image viewer / light editor I’ve used.
For Numbers I like gnumeric over libreoffice Calc, but nothing really holds a candle to excel unfortunately.