but my understanding was that the spice was always needed for interstellar travel

No. Here is the timeline of Frank Dune (that is, without Encyclopedia and Brian). Spice properties was discovered during the Shakkad the Wise reign during the Old Empire (pre-Butlerian Jihad), so the vast interstellar polity already existed. And while it might theoretically be possible without FTL due to long timescale (for example Dune is 300 light years from Earth so 6000 years would be enough for Muadru to arrive), look at the timescale of Zensunni pilgrimage which happened before discovering spice and which took only 80 years through multiple star systems. Meaning even exiles and slaves had access to FTL in some form. Though of course it was probably method slower than instanteous teleportation using holtzmann drive, and most likely involving extensive computerisation. Also there were 2 space migrations in barely 200 years happening at the beginning and it was enough to apparently spread quite much since Jews needed to hide after that period implying that a lot of planets were already settled in 200 years.

it was the spice monopoly that forced everything in to stasis, made the Frefeluchs system possible, made the dominance of the Emperor and the LANSRAAD possible

Not specified, but the guild monopoly was the consequence of jihad, since banning of computers made FTL crazily risky and basically only the navigators could do it safely. Brian Dune makes creation of the Guild explicitly consequence and last step of building of the Great Convention status quo, when the Venport Holdings corporation forces were destroyed by the Empire and Butlerians, and forced to conform to new order.

The stasis came from the universal economic dependence on Arrakis, the spice, and the spacing guild. Even in the worst case of rebellion the rebelling worlds could be contained by embargo indefinitely.

Yes but all this was the consequence of outlawing technology, because it forced complete dependence on spice for FTL. Note how after Leto II death scattering loosen the Jihad contraints and it leds to noticeable inprovement in overall humanity condition in mere 1500 years, in Heretics and Chapterhouse there is a lot more technology mentioned (including even things like cybernetics!) than in the previous books. In GEoD there is openly stated than nearly entire humanity is reduced to the level of subsistence farming (this is btw point when something don’t compute hard since how the hell isolated subsistence farmeres would even experience empire-wide famine or construct spaceships but it was never mentioned, even Bene Gesserit with their memories never mentioned anything about the times except it was horror).

(I mean the real reason for banning tech from Dune is to remove technobabble which cause most books to be obsolete and funny in two decades, especially that Dune do contains few examples of really ridiculous fantasy tech, like teleporting huge ships or low tech and low energy antigravity)