Is the Reach less defensible than the Riverlands? It’s larger and has two disconnected river systems, they border five other provinces, and have (what seems to be) a largely unwatched maritime border. It seems like nearly all of their naval assets are concentrated in the Redwyne Straits, and that if anything happens to the shield islands there’s nothing left to secure the Mander, and it doesn’t seem like they have an actual riverine navy to compliment the Arbor fleet

No, it’s actually significantly more defensible. It’s got the Red Mountains guarding its southern border (and to some extent eastern) and some big beefy castles guarding the passes, rivers which narrow approaches on its eastern borders, and the hills of the west funnel the Lannisters down to the coast road or the gold road, which mean getting past the Oakhearts or the Rowans, and giving the Tyrells time to mobilize.  

We shouldn’t let Euron’s raids blind us to the true naval defenses of the Reach – the Shield Isles are substantially stronger than Seagard, just ask Quellon Greyjoy, and the Hightowers also have a substantial navy (see the Sea Lion).