You have some interesting suggestions listed here, but it's not clear if you actually tried KOReader's built-in
k2pdfopt library.
When reading a pdf, you click on the lower part of the page, select the 'Aa' icon, switch 'Reflow' on, and choose a magnification level.
The k2pdfopt will then split and magnify your lines of text, with varying success.
Images and figures cannot be split, so you could try using KOReader gestures to quickly switch in and out of Reflow, say, if you need to zoom in to see details of an image that has been scaled down to fit on the page (Zooming images doesn't work well in Reflow mode).
That could admittedly get annoying if you have a lot of big images - in that case the preprocessing suggestions may do the trick.
Only then would I splurge on a) a big screen that you don't have to zoom, or b) a powerful device that zooms and pans very fast!