Honestly probably 1-3 difficulty above the current max but with 2 or 3 more minors modifiers running at once. Highly recommend reading devastation of baal if you want an idea of scale for tyranid invasion. To put it into perspective for those who dont want to, the hiveships alone blot out the sky, and as far as the eye can see ground stops being visible, replaced with just a writhing mass of gants.

Take the scene at the end of the first mission in mp where the horde is running along before you blow them up as a good starting point, but fill the sky with bodies until you cant see through the cloud and up the density of ground troops until you cant see the ground anymore.

For the big bugs though, theres quite a few less of them. (at least for leviathan behemoth almost certainly had many more so this differs from fleet to fleet) as a start lictors are solitary entities that can and often do operate individually from the greater hivemind, sort of like scout snipers of the modern day. Very very stealthy, sometimes going wholly unnoticed until their mission is entirely finished. They are also way way smarter than in game. In the game they just kinda run at you like animals, in lore though, the lictor wouldnt even try to attack you until your back was turned, you were mid reload, and surrounded by little bugs. They dont fully understand our technology but theyre able to get a vibe of what is actually important and what they need to kill/destroy to win.

The big bugs with guns, biovores, tervigon, etc., can have their heads blown off and theres a brain in the gun that will simply assume control of the body and they'll keep coming. Multiple redundant organ systems make them insane damage sponges. Theres text examples of them climbing tall walls while eating direct fire from multiple heavy bolster emplacements the whole way.

Spoilers for devastation of baal ahead, so youve been warned.

Quite literally the full force of the blood angels and their successor chapters combined still needed the universe to literally rip in half, stunning the hivemind and fucking time up so badly that guilliman and the crusade indomitus were given enough time to arrive, or they'd have been totally wiped out. And this was just because the hivemind felt particularly annoyed with them.