Why is this even in aquascaping?
probably would get more responses in the general forum. Anyway, as a self professed slacker that lets his tank go through fits of water change neglect nearing a month *cough 6 weeks cough* sometimes, I'll point you to what I think is the most important aspect of tank survivability in this scenario: light.
recall from the three pillars theory that light is the gas pedal that drives everything. lessen light, lessen consumption -- from a plant aspect. So, since you smartly asked about this at least 4 months ahead of time, I'd suggest you start practicing now. Lessen the light somehow (let the floaters go wild, lessen the times, whichever) and stretch your water change to 3 weeks and then 4 and so on. You can also recalculate your ferts to stretch that long -- or simply stop adding ferts once you reach your estimated level as your plants will not be consuming them as fast. Don't touch the co2. Just make sure your floaters don't choke out that surface agitation in 6 weeks growth. As for the fauna, since you have nice waste collectors in your shrimp, and "a few smaller fish," it doesn't sound like you have a huge organic load, you might just be able to pull this off as long as you figure out how not to overfeed.
I don't have a picture to go by and you didn't mention what kind of plants you have. So, without more specifics, I believe the above is the general idea. Will sensitive plants suffer? You bet. But at this point, the goal is to keep them alive until you can come back to pamper them. Will tough plants go crazy with no trimming? You bet! It's a great learning experience to see what plants are great for your tank. Are you just raising java fern? Then stop worrying.. those guys survive anything!
Just worry about overfeeding the fauna!