Worth parking some Rare Candy with Chansey? In what order (and on which Chanseys) should I use my Chansey candy?

If you're going to evolve a Chansey, evolve #1. There is no HP difference between a 14 and 15 stamina stat on Blissey.

I would suggest evolving #1 and using #2 as your Chansey. They're high IV. Personally, if you want to max one, I would invest in Blissey #2 (Chansey #1) and make sure it has ZH/DG. I personally have no problem investing RC in 5 km mon *that I plan on using*, but different people feel differently.

No rare candy for chansey. Put more rare candy into you numerous level 30 and potentially level 30 legendaries.

Level 30 is a good point to stop for chanseys/blissey before the costs go really high, and you can maintain a fleet of them.

No.
Rare candies only on legendaries

With your resources I would not invest RC in Chansey. Investing in top tier attacker legendaries pays out in more damage balls ie. more RC in the long run. Chansey/Blissey can only indirectly help you get more RC (by keeping gyms before a raid starts).

Make Chansey #1 into a Blissey and take it and Chansey #2 to level 30, you're only 2 candy short for that. Then just be patient and power up your best Blissey as you get more candy from walking, feeding and quests. If you're dead set on using RC, it's better to take both of your Blisseys to 3k cp than to max one of them.

For reference, I started putting RC into Chansey when I was well beyond 40m exp with one of each relevant legendary maxed. I don't think you should consider it before your attacking squads are in really really good shape.

I am not in a high turnover area like you are so my situation differs but I will tell you how I played it and am still playing it. I have 2 ZH/DG Blissey. One is at lvl 35 and the other is lvl 32. I am a lvl 37 trainer. I did use some rare candy on my first Blissey but I have since stopped. I don't think there is a set in stone rule for rare candy, I just now prefer using it on legendaries. Once I get my second Blissey to lvl 35 I am probably going to stop and begin working on either a Blissey or a Chancey. My advice at the end of the day is work the 2nd Blissey and then make the call about the rare candy based on what you think you need.

You can use the SG EX Raid telegram to find and spin those “Hatch 5 eggs” quests for Chanseys.

No.
With the SG EX Raid (Quest) telegram group and the relative abundance of “Hatch 5 eggs” quests in SG, there isn’t a need to use rare candy on Chansey.

I used 30 rare candy to max my first Blissey (15/13/15) a few months back, before quests were introduced. Back then, it was very satisfying. Luckily for me, I didn’t incur any opportunity cost from the rare candy used on Blissey. But I would advise you not to do it.
This blissey is now maxed at CP 3205.

I also have a L33 Chansey, 60-odd IVs, caught weather boosted during Valentines Day. (Admittedly with sgpokemap.) I’ve been slotting it into gyms as it is. Never powered it up.
It had ZH/HB at first, I TMed it to DG when I had 10+ CTMs. Thankfully it took only 2.

Now I have a second Blissey (15/15/14) at level 37, CP 3078. When maxed to 40, it will have CP 3215, and “practically perfect”, having the same HP as an actual perfect Blissey.
But I’m in no rush to max it now, I’ll just slowly do it as I get candy from:
Hatch 5 eggs quests
Feeding the Blisseys/Chanseys in gyms (especially those I’m targeting for gold)
The occasional Chansey hatch
I also walked Chansey/Blissey as buddy for most of Adventure Week.

If that's your goal, then start walking Chansey, or Blissey. Rare Candy is for Legendary Pokemon. Anything you can walk at 5km per candy isn't worth it.

Further, if you're level 38, and your strongest pokemon, like Groudon, Raikou, and Lugia, are only at level 30, then you might as well be level 30 and not level 38. You can power Groudon all the way up to 40 right now. Raikou too, and it's relevant right now as by far the best Kyogre attacker. That's what rare candy is for, not for bulking up Blissey and Chansey to get a few more coins.

So my vote is no. Work on getting a Kyogre powered up before Groudon comes back, or save some RC to get a good Rayquaza, which will be useful against Palkia. Don't waste RC on non-legendaries, even Blissey, Dragonite, Tyranitar, or whatever. Just not worth it.

It's pretty inadvisable to drop rare candies on anything non-legendary. Power some legendaries up past level 30 so you aren't just level 38 on paper.

Powering anything up to Level thirty and keeping it there is a flat out waste of resources. Any non-legendary can be caught up to level 35, even if you have a 100% dragonite at level 30, a lv 34 Dragonite with 70ish% IVs will almost certainly perform better. In order to really benefit from that improved IV stat, it should be powered at least to 35 or higher.

As for legendaries, in most cases they are flat out better than most non-legendaries (looking at you suicune). But again in many instances (kyogre and groudon excluded) you can't realize those benefits unless they are powered up higher than Lv 30. Why would you want to spend 75K dust and 66 rare candies for a Lv 30 raikou that is effectively a Lv35 jolteon which can be caught on pretty much any street corner for little more investment than time?

If weather boost didn't exist and max level for wild caught pokemon was still Lv 30, then fine. It's a cheapskate way to play but you weren't going to catch anything better than that. But that simply isn't the case anymore and you need to adjust.

That said, putting rare candies into ANY non-legendary when ALL of your legendaries are below level 35 is poor resource management. Are you really going to go walk your mewtwo 20K for 1 candy? or 4960 KM for a max mewtwo? If you ARE willing to do that, then you could've had enough candy for 4 maxed blisseys by then. Ask yourself am I REALLY willing to spend a dollar for 25 cents return?