So in 5e with its flat power curve you should resist making the bottom tier be CR 6.
As a good approximation, CR adds up in 5e. So a CR 6 monster? If there are 4 of them, this is as nasty as a CR 24 monster (roughly).
By making the least of them be that tough, you limit the ability to have more than a few of them around (combat encounter wise).
Drop then to CR 1 or 2 and you can have swarms of the least ones. CR 3 for beefyones.
CR 3 - Fledge
CR 6 - Haunter
CR 9 - Flesh Crafter
CR 12 - Fossil Lord
(Note that above 20, CR "costs" twice as much)
This pattern lets you have a Lord, Crafter a Haunter and a half dozen Fledges as a deadly encounter for T3/T4 PCs.
I see you have put the Fledglings at CR 1. That works as well; more swarms.
CR 1 - Fledge
CR 4 - Haunter
CR 8 - Flesh Crafter
CR 12 - Fossil Lord
Again I top out at CR 12, as (to me) I feel this isn;t a dragon-type solo but more of a comtroller of a force of these critters.
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I personally would then start with my
D&D 5E - Ridiculously Simple Monster Building
18 MBP - Fledge
30 MBP - Haunter
46 MBP - Flesh Crafter
62 MBP - Fossil Lord
Keep it simple in iteration 1.
Fledge gets 16 AC (6), 24 HP (3 MBP), 16 in attack stat (3 MBP), 18 standard attack damage (6 MBP).
We'll make it incoporeal - immune to non magical BPS, resistant to magical, immune to cold/poison/acid, resistant to fire/thunder. And half its HP to 12.
It emits a sphere of diameter 5' of magical darkness they can see through and suppresses nonmagical light within 50' down to a 5' radius. This is a weak form of invisibility (worth 4 AC).
I'll make it strength based. 16 strength, 14 dex, 12 AC (+4 from perma-darkness above).
Now for damage. Body horror sounds fun. How about acid damage that lingers? 1d6+3(6) slashing plus 2d6 acid (7) is 13. We have budget for 18... so, make the acid linger.
Acidic Talons: +5 to hit for 1d6+3(6) slashing damage +2d6(7) acid damage, and a 2d6 of acid clings to you.
Clinging Acid: You can safely remove clinging acid from yourself or a creature adjacent to you as an action; make a Medicine check using a Healing kit. Every 2 points above 10 safely removes 1d6 of clinging acid.
You can also wash it off using 1 pint of water per 1d6 acid, but doing so causes you to suffer the damage as it reacts to the water.
If you don't remove it, at the end of your turn roll your clinging acid and suffer that much damage. For every acid die that lands odd, the acid wears out. If you gain clinging acid when you already have it, it accumulates.
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Now add on movement (no flying), and other stats etc to taste.
We now have these pockets of darkness monsters we can swarm PCs with. The acidic claws give a bit of a body horror feel the delayed damage adds to fear.