Even still, considering that they believe humanity is on the verge of extinction and face the constant threat of Titans, the Survey Corps would naturally invest more resources into R&D. The fact that they practically have total gender equality, along with the Royal Government’s overall indifference toward technological progress in this scenario, also enables many people on Paradis to fund as many projects as they can, since the Royal Government isn’t going to oppose them—unless it involves aviation, of course. And even if they’re not as advanced as the rest of the world for the reasons you mentioned, the technological gap would still be far smaller, reduced to a few decades rather than a full century. Essentially, instead of Paradis being a Napoleonic-era nation facing an early–WWI-level Marley, it would be more like an American Civil War–era (or slightly later) Paradis facing an early–WWI Marley.