John,
There's really 2 things going on here ...
One is overcoming the physical limitation of a printer, and
The other is image quality for a given file size (in terms of pixel dimensions)
If one needs to chop up an image to overcome the limitations of their printer then there's no way around that (short of having it printed elsewhere) (so I won't comment further on that), but my real issue is with the 2nd part - especially where you say "My camera specification states maximum print of 20"x30" up to ISO 400"; this is just plain WRONG ...
... you can print as large as you like - from any quality source - and at any ISO; it's simply a case of "what will the minimum viewing distance be before the degradation in image quality becomes visually obvious".
I hear this 300PPI bandied about all the time and it's just crazy crazy crazy. If one prints a file much larger - so that the effective PPI drops to say - 200 PPI then you'll have a much larger image - which will then normally be viewed from further back - where your eyes won't be able to resolve the retail.
The short answer is - just print the darn things as large as you like - forget what the "specifications" say - and stop looking at them up-close with a magnifying glass.
If your wife asks you to vacuum the lounge, when she comes to check on how well you've done it, would you expect her to stand at the door and look, or would you expect her to pull out a magnifying glass and go over every square inch "up close"?
Same for photos!