Which External Linear PSU Upgrade for my Router?

W=VA

12V is your given and you need a power supply that provides "at least" 2.5A at 12V, so at least 30W.

Sbooster power supplies are fairly well-respected and this one would work for your situation.  The 12V model provides 3A, which is fine for your needs.

Ifi power supplies are not linear supplies, but they are designed for low noise and many seem to like them.  This 12V model provides 4A, which works for you.  This model only provides 2A at 12V so wouldn't provide enough power for your needs.

Fidelizer Audio Nikola LPS. I own their Airstream - used as an access point after my Nighthawk. I also have 3 of these - along with multiple tweaks for cleaning power.

A router puts a high demand on an LPS - especially for streaming movies and gaming. Be sure to get one that has a decent heatsink. Unlike a DDC or DAC which has a minimal load and barely gets above room temperature.

YesI got this started long ago ,call linear tube audio they have a great LPS 

retail $750 you just tell them the voltage. Their LPS can go up to 8 amps

 routers - modem combo are around 4 amps at most.

What is the goal in using a linear PSU as opposed to say, using a power conditioner?

I'm using the exact sbooster model as pointed out above on my router. Can't say I hear a difference, it was an extra power supply I had lying around is why I'm using it. Makes me feel better is all.