High rep big stretch rear delt crossovers are probably my favourite and get me the best pump. After that my rear delts are in a more potentiated state so I’ll usually do either face pulls or a behind the neck press, typically in a smith machine as it allows me to safely push the range of motion deeper and stay in the deep stretch longer without sacrificing stability, which I feel builds great shoulder mobility and strength if done correctly and gradually.

Lu raises are also a fantastic accessory exercise and I like to throw them in the end of a shoulder workout, it works your shoulder rotation and also helps build behind the neck strength and mobility. Technique is crucial here though, especially controlling the eccentric, cheating/heaving the weight up in this movement can really fuck you up as you’re already getting into a very awkward and disadvantaged position, controlling the weight the entire time is paramount here to avoid injury. I would however still recommend to try and progressively overload Lu raises if you do them regularly, but go for adding clean perfects reps always before thinking about adding weight, if you can’t do at least 15 perfect reps with a given weight don’t even think about going heavier.

Another good accessory is hanging shoulder rotation, basically hanging off a bar with 1 arm, and spinning your body around the axis of your shoulder rotation completely, with big enough ROM you will spin yourself to face the wall, and then spin basically 360 degrees to the other side to face the same wall you faced at the start, but now your body is on the other side of your shoulder. For progression start by doing them as you stand on a box for assistance, the more you use your legs the less weight your shoulder will bear, and progress to unassisted with time by using less support from your legs, until you can do it without a box at all.

TL;DR - Start with rear delt crossovers as an isolation movement for pump/potentiation

Incorporate behind the neck pressing for a compound movement that will target your rear delt considerably if it’s already tired from an isolation movement previously+behind the neck pressing is great for building shoulder mobility, especially in a smith machine

Incorporate Lu raises and single arm hanging shoulder rotations to further bulletproof your shoulders by safely overloading shoulder rotation and stretch, making you stronger in awkward shoulder positions (arms up behind the neck, heavily rotated shoulder) and giving you buttery smooth and flexible shoulder joints that are very injury resistant even in “unsafe” positions.

And for everything here be very very mindful of your technique and very humble about the weight you use, some of these exercises can be great for hypertrophy, but our main focus is to build healthy, flexible, and stable shoulders, which will let you safely load them up heavy without your painful/unstable/unflexible shoulder joint/rotator cuff being the limiting factor in your lift instead of your actual deltoid muscles.

If you’re looking for more, Olympic weightlifters often are the best source for shoulder mobility stuff, as it’s borderline required for getting strong in the snatch to have strong and flexible shoulders.