If the foliage is thriving, maybe let the plants acclimate to the new situation for a while without cutting back. I have some spindlies also and learned they are hard to deal with. If desperate, you can cut back "hard". This will cause them to either spring into growth below the cuts......or die. Ideally, one would prune to just above a dormant bud to encourage fresh, new growth, but these, if present, are hard to find/identify on old wood.

Going to youtube and searching for "rhododendron" will give you much pruning info.

Good luck!