I'm 90% sure Eldrican is waiting for us to 'invade' their ally, so they can invade us.
I simply don't believe your analysis of Eldrican motives.
You said the same when you thought it was only 4,000 troops and couldn't be a threat except as a feint. Now it's 30,000 troops lining up but I'm so glad to hear this ISN'T the promised Damdamian Crusade. It's still just Eldrican machinations to make us look bad.
Okay, I phrased that wrong. Eldrican isn't waiting to invade us. Rucus is waiting for an excuse to call Eldrican.
At this point it is a full crusade. No denying that. Eldrican does not like us and they have a formal alliance with Rucus/Lamentia. I don't think they are mobilizing though (we'd be getting intel about Eldrican mobilizing if they were mobilizing). I don't know why; Perhaps our rotten cousin wants his friends to do all the work, before swooping in to take the crown after the war. If we invade their ally, they might not have a choice but to get involved.
Look at it from the crusaders perspective; We have a massive, well trained army that is mostly defensive heavy infantry on the border. We have nasty border forts. A good chunk of that border is river, if I recall correctly. Unless they are stupid, they are not going to simply rush across the border. They need a scheme. Getting Eldrican involved is the most probable scheme.
Stalling also doesn't favor them. Crusaders, historically, are not the most well behaved people. I would not want a standing army of 30k foreign zealots hanging around my territory even if they were my allies. Also, if they take too long, we might get opportunities to try calling more allies or use our spies to fuck with them.
We have a troop boosting artifact
They have religious fervor.
a training edict
Past Damdamian armies have had equal training thanks to the legacy of the former Empire. At least some of the banners reported are those of national armies.
probably superior equipment (Damdaimian lands are mineral poor), probably superior fireblood weapons (we have a fireblood court), recently revamped and modernized forts,
I'll grant these, although I note the probably in there (see military legacy of Empire, above).
and a Martial Leader.
It would hardly be the first time our opponents do as well. The more armies assemble, the more likely some of the commanders are equally competent.
It would seem that you read up on the time Houlandin I attacked Rucus/Lamentia. In case you don't recall, WE LOST THAT WAR! I don't see how that goes better for us now that they have a bunch of foreigners helping them out.
The way I see it, it breaks down three ways:
Them attacking without Eldrican - favors us.
Them waiting - favors us
Us attacking without more factors in our favor - favors them, and probably drags Eldrican in to boot.
I'll sweeten the deal by stipulating that I have no desire at all to conquer and keep Rucus. Swat them down, bring everyone back home. Let's repurpose our diplomats and spies to spreading that message in southern lands and maybe some of the rulers down there will obliquely reward us for removing a troublesome fanatic element.
If we do invade and win, we need to give those lands to our Vassal Eval. Historically, they have a claim to Lamentia, and likely have a similar claim to the former duchies of Lamentia that Rucus has annexed. Praticailly it would create a nice buffer state between us and further Damdamian aggression.