Investing in another defender

Anything you can power up can return in minutes. Nothing you can do about it.

While that is a very nice IV Blissey (Congrats) I'd say in the current [attacker-centric] meta it just isn't worth it in MOST areas; you would know best about your local trends.

My opinion is if you don't have issues getting 50g most days then save your dust.

Better to make 4 level 20 blisseys than 2 level 30 blisseys.

No.

Don't invest in Defenders - at all. You could max out a 100% Blissey, put it on a gym, max out the CP by feeding it berries, and still have that Pokémon come back before you earn a single coin. Or you could have that defender on the gym for 7+ days. It is entirely, 100% dependent on the play of other players and the only thing you can do about it is to make friends with other players in your area and make agreements to wait 9 hours before taking down gyms.

And even then that won't stop the assholes who won't abide by those "gentlemen's agreements" and will take down gyms whenever they feel like it. Got a gym a block over that's been held for 1 day? Too far to walk - I'll just take down this gym right here that someone else just took down 10 minutes ago. Eh, sure it means 3-4 battles, but who wants to walk a block? Nothing you can do to stop those assholes.

So don't bother investing in a defender right now. Hopefully there will be a gym rework to make defending more useful (I'd LOVE it if Pokémon could increase their IV's by being on gyms, for example) but for now your best bet is to simply put as many Pokémon on gyms as you can and hope that they can earn a combined 50 coins a day. Even if you're tossing on a Shuppet you caught 20 min before the bus got next to a gym with room on it and you only have time to put a random defender on as the bus pulls away - that Shuppet might earn you more Coins than your meticulously TM'd and powered up Blissey Tank.

I'm giving you a different opinion than above. Blissey is still a pain the ass to fight against. These days no one is short on potions/revives and most players will be able to beat any Pokemon.

The only thing that is valuable is time. Players want to destroy a gym as quick as possible. Even tho it's not gonna prevent players from taking over a gym, you will still waste some time and that what counts (for me).

I'll continue to invest in Blissey and I'm intending to max out my 100%

It is worthwhile to have at least one Blissey, Snorlax, or Chansey powered up to hold a gym before a raid starts. Beyond that use though, I'm not investing too much in defenders.

Blissey is obviously still the best defender (at least short term), as it takes the most time to defeat. But there might be a Blissey crusader in your area - someone who is sick of them and wants to dissuade people from placing them.