As I said:
Threebond does have a good reputation & I’ve used it, but on the very bottom end, I’m not convinced it is any better than the sanitary silicone sealant that I’ve used for years. If I still had Golden, or even Red, Hermetite to hand I would use them on the base.
My unconfirmed, unscientific belief is that Threebond is a product that may work well or better @ the elevated temperature that are encountered @ the top end of the cylinder where you can accept, or prefer a bonded seal. I used to use (wait for it!) Green Hermetite there.
However, with the changes from hot to cold as an engine runs & then stands, inevitably, there will be some movement, or ‘creep’ that must be tolerated somewhere to maintain the integrity of the whole assembly.
My preference is to acknowledge that need by utilising a compound on the base that has some degree of flexibility to allow for that movement.
IMO the Red Hermetite would be my choice from what you have available.
Seriously, the silicon sealant that smells of vinegar (preferably the low modulus, more elastic type) is incredibly well suited to sealing tasks that need to accommodate some movement without failing. Sceptics cannot understand that it can cope with the temperatures encountered (I use it on the exhaust flanges) but when you consider that silica is the most common component of sand, you begin to understand why. I NEVER use Blue Hylomar. I'd sooner use Cow Dribble.
In all of this, I’m just trying to offer a little sideways advice from what is the accepted norm, to help you reach a solution. That leaves me often vulnerable to criticism, but I wouldn’t claim to be an expert @ anything.
I can only suggest what I would try….
As for:
I have to admit to being slightly nervous about 30Nm plus Copaslip ; I reckon that's equivalent to about 40Nm dry! All my threads are brand new though, so I'll give it a go and take it up little at a time.
If you can ensure that the compounds utilised to seal are the optimum, then you should not need the kind of torque you anticipate.
The thing I should empathise is I advocate some lubricity between the cylinder head nuts & studs that will remain despite the cycles of hot & cold, which Copaslip will do. This facilitates re-torquing if required. A tiny dab is all that is necessary.
My own practice has been to studlock the studs into the crankcase, often V*spa rolled thread studs with as deep as an engagement as possible. If threads (rarely) pull, I am fortunate enough to have Helicoils which many engineering manufacturers insert @ manufacture, rather than just as a fix.