I think there's an Occam's Razor answer that the Empire could have reconquered Lothal very easily but the Emperor had it put on his Death Star list.
Because unlike Lothal, the Mandalorians were a symbol.
On the other hand if you plot out the seasons and the time jumps starting around 5 years pre-OT, it neatly puts the end of season 4 right around the time of RO.
I'd also argue that it makes more sense for the Empire to wait for the Death Star to deal with Mandalore than it does Lothal. Mandalore after all is fortified, heavily armed, and filled with proficient soldiers. Lothal mostly just had a small Rebel force armed with a modified light freighter, and grass. Lots and lots of grass. Bringing down Mandalore conventionally would have taken a significant commitment of hardware and personnel. Glassing Lothal would have taken just two or three destroyers. So yeah, I tend to err on the time-span between Lothal's liberation being smaller rather than larger.
Honestly, just looking at it from a storytelling perspective, it seems way more interesting if this all happened in pretty quick succession; loosing Thrawn, the 7th Fleet, the TIE Defender Project and the Lothal Sienar factories; then less than a week later you have an open attack on not one, but two supposedly secure weapons research facilities, followed by the Death Star's destruction, the loss of Tarkin and the sheer chaos that likely followed both in the Empire at large, and the upper echelons of the regime. It'd be a hell of a signal to the galaxy that the Empire's grip on power was slipping, leading to open uprisings across the galaxy and setting the stage for the Mid-Rim Offensive (admitted followed by the Mid-Rim Retreat and later the route at Hoth, but still!)
Dark Jedi are certainly interesting villains, but again they are just too few to even bring this to a Galactic scale problem.
Feels like Star Wars is shrinking to small local conflicts.
Well you can't exactly have a galaxy-wide invasion and a rogue super-weapon ever other week. This isn't the EU after all! That said; the stakes of this show are very clear; if Thrawn returns a galaxy wide war may be exactly what follows, so best work to prevent that when when it is just a handful of rogue nutters, and thus rather more manageable.
Regardless; you don't need massive galaxy ending stakes to tell an effective story. What matters is the characters and how their stories play out. If you're framing this as a story about people getting into cool fight sequences, then you're looking at it all wrong; it's about people.