Chansey as defender

Defender viability depends completely on your local context, down to individual gyms - how determined, capable, and frequent are the attackers? In some of my neighborhood gyms, a 10 CP Pidgey is just as viable as a max Blissey, because either one will sit there for days. At the other extreme my nearby gyms with high foot traffic, even a gym with a maxed Blissey and 5 other good defenders won't last more than a couple hours.

Chanseys are somewhat interesting in a middle zone, where attackers come by rarely but definitely, so you need mid-CP defenders (roughly 800-1600 CP) that don't fade so quickly.

But even then a 500-CP difference in your Chansey isn't really going to deter attackers. Basicallly investing any stardust in defenders is currently not a good investment, once you have at least a couple/few high-CP Blisseys and Snorlaxes for the high-pressure scenarios where it's hard to hold a popular gym for at least an hour or two.

I recommend saving that dust for your Tyranitars, Dragonites, and whatever ideal attackers are needed for your tier-4/5 raids, so that you can continue to do high-tier raids as the number of players dwindles through the winter, meaning we'll all need multiple high-CP ideal attackers to finish raid battles. You'll burn 340k dust in a jiffy in order to power up, for example, 4-5 of your Raikou in order to take on Suicune with just 3 other people.

Yeah I think the Chansey is fine where it is. Frankly, for most people Chansey isn't much of a deterrent by itself, as part of a fully stocked gym (with a Blissey, Snorlax, and a few other bulky defenders) it could give people pause, but most trainers are able to take out a chansey (regardless of level) without much difficulty, so honestly I would just save the candies until you get enough to evolve.

Don't power up chansey. Just keep as is. Still good for wasting other trainers time. For example if someone wanted to take a gym and it had level 39-40 blissey snorlax chansey wigglytuff clefable dragonite or whatever, I would just shave one pokemon out at a time so they dont have time to react and use golden. If someone wants the gym they will take it. A level 20 chansey won't be different from a level 39-40 chansey. It might waste like 1 more minute total but that's about it. Plus a stock chansey freshly hatched will last a long time so cp won't decay fast like a 3200 blissey

Don't power them up. Find a gym you hope to hold for 24 hours, and toss them in there. Their CP goes down more slowly than a Blissey, and they are more of an annoyance for attackers, but they do deter casual attackers.

Look for high CP wild Chansey. They can be caught as high as level 30 in the wild. IV's aren't that important, I have caught a couple over 1000 CP. You can also transfer them after they get kicked out of a gym to save revives and potions.

I dont care what anyone else says. I dont have a Blissey yet but when there is as 3000+ iv Blissey in a gym I walk away, just too damn long and annoying