Hi Lewis!
You asked for thoughts so here are some...
I have not used the combined product you linked to, but do have separate...
Battery lighting (LED blinkys)
Dynamo lighting (steady headlight and taillight)
Alarm
...on most of my bikes. The alarm is a separate unit that mounts under my saddle and uses a several AAA cells and a separate remote for dis/arming and alarm/sensitivity selection. I got mine off eBay and they (I have 6 at present) continue to work well a couple years after purchase. I use Eneloop rechargeable batteries as they lose less charge in storage and don't leak. These replace earlier models that mounted to the seatpost and required keypad entry to dis/arm and set sensitivity. They worked well for several years but ultimately died and it was a hassle trying to set them without a remote.
The =idea= of an alarm has worked well for me in practice the way I use it. I installed my first motion-detecting alarm on a bicycle back in 1987 when I was commuting regularly and needed to leave the bike parked in racks. My office was nearby and I could hear if it was triggered. I set the sensitivity to ignore small bumps but sound if a larger bump was detected, thus minimizing false positives which legitimate rack users found (and reported to be!) extremely irritating. Once I got things adjusted and parked at the far end of the available racks, all was well for everyone concerned.
Now, I use the motion-detecting alarms for my day rides and touring to alert _
me_ to problems. For example, in summer I regularly ride a 108-mile training loop several times a week. At the far end is a portable toilet too small to bring the bike inside. Though I set the frame's ring-lock, it is nice to have the alarm on also to tell me if someone is tampering with the bike while I am indisposed and might not notice otherwise. On tours in the mountains, I regularly set the alarm to deter small animals that sometimes climb on the bike in search of food. Porcupines in particular seem to love the taste of leather saddles whether they are under a cover or open, and the alarm has deterred several that were on their way to taking a nibble two. Similarly, I have triggered the alarm with my uniquely paired remote several times in the last year to deter bears I could see wandering into my campsite while I am abed. It works nicely for that, causing the bears to sort of compress mid-stride before turning tail to flee what seems to be an infernal and unfamiliar sound (I always bag and hang my food apart from my tentsite whenever I'm in bear country but really don't want to encourage a close-up visit). When I tour and camp in more populated areas I set my alarm and use my frame mounted ring-lock and a plug-in cable or chain as desired.
My general experience with combination gadgets is they don't do as well as single-purpose gadgets. I suspect that most of the price for this item is based on the convenience of having both and they docking/activation function, which could be really handy, though the light might be bulky to carry with you. The website indicates...
The SR600 can be paired with multiple other units so you can share its security with family, friends and even clubs.
...but I think you would need multiple alarms and mounts to work with one headlight/switch. Perhaps there is a discount for the one piece?
For the most part I try to avoid devices with embedded rechargeable batteries. I have found their service life sharply diminishes after a couple years' use and then it is either impossible to replace them or recycling is problematic, so I prefer user-replaceable batteries and choose to use Eneloops for the reasons mentioned above. Keep in mind, most remotely triggered alarms continue to draw current on standby. The one you linked to has a very good claimed alarm standby time of 10 days when new.
One last note on the alarm/light combo: I have found my alarm - though loud -- draws surprisingly little attention from others. People seem to be either inured to alarms in general (i.e. car alarms) or do not want to get involved when a property crime is occurring. I consider my alarms to be for my own notification and they have worked well when used in that way.
You asked...
Would it be realistic / safe to take a lighter bike lock than my usual D lock??
I think it depends on where you ride and whether/how long you will be leaving the bike unattended. I live in an area where bike theft is endemic and there are now snatch-thefts where bikes are taken with riders standing next to them. For rides where I don't intend to leave the bike alone but might stand beside or near it, I take a small cable and combination lock I thread through one or both wheels and around an object simply so the bike cannot be snatched while I am peeling off my leggings, removing my rain booties, or taking a photo. Apart from a portable toilet, I take my bike into public restrooms with me and use a similar means to secure it while I'm indisposed. If I have to leave the bike alone for any time at all, I remove the front wheel and lock it to the rear with a U-lock and large cable, but I much prefer my frame-mounted ring-locks on the bikes that have them. It is so simple to lock the rear wheel simply by throwing the hasp, or depending in need, I can carry and use a cable or either of two chains (6mm or 8mm hardened links) depending on the amount of security needed. Of course, the downside is I am always carrying at least the weight of the ring-lock on the bike and this makes a light bike heavier than it would otherwise need to be. For this reason, I use the little cable and combo lock on my Fixie, which is my lightest bike. Out in the countryside where there are open fields or in the forest or desert, I don't have to worry and usually don't employ any security except my alarm as protection against animals bothering the bike or its contents. YMMV.
All the best,
Dan.
