For trailer security, a good padlock will be required for the rear doors, the sturdier the better to slow people down. To prevent the trailer from being stolen, forget padlocks on the tongue. Tongue ball locks are good, but people can still chain the tongue and tow them off to a corner to bust open the doors. Wheel locks are best. A couple of different types out there. The one I have is this one that I got on sale from PA which they occasionally have on sale for under $40. https://www.princessauto.com/en/heavy-duty-trailer-wheel-lock/product/PA0008196735. It's mildly compact for storage. Make it a habit to use them everytime you park and click them up as tight and snug as you can as they can be defeated if they want to fully deflate the tire to try getting them off the rim, but then they'll need to re-inflate to drive off. Park with the trailer and both locks in reasonably plain sight of people or a security camera so if someone tries to use a cutter the sparks will attract attention.

That Volvo OEM trailer wiring harness (shown in some green manuals) was special dealer or factory order to my knowledge and not cheap. It connected up under the dash and went down the left side to the rear.

As for highway speeds, their 55mph is just a serious suggestion and may even be on a sticker for others to see. That's 90 km/hr, which covers most of our highways here. On the odd freeway, you might feel comfortable taking it up to maybe 100-110, and much above that you'll want to be a lot more careful. You have to be careful about trusting the integrity of their used tires and wheel bearings at higher speeds. A blown out trailer tire can be a major pain. Be sure you're packing an appropriate tire spanner, cheater pipe, scissors jack, wheel chocks and road marker. Gas mileage suffers exponentially at higher speeds with heavier loads and increased wind drag from a trailer, so best to take that kick back and relax attitude.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now