On sidemount, most view hybrid (handling backmount and sidemount) systems to be less optimal, and more frustrating. If you go sidemount as your entry to tech, pre-CCR, I'd just plan on a dedicated sidemount rig. Like say the XDeep. Not a hybrid! It can be dove sidemount with one or two tanks, or 3-4. With a CCR in the picture, the cost of that rig is lost in the noise. (They can come in colors, SumpUK has nice hardware to pimp it out)
Backmount tech involves dealing with stages, which are a lot like bailouts. The differences between sidemount tank rigging and stage tank rigging are not that complex.
I would not expect to travel with manifold doubles, but rather for the boat to provide them, assuming it is a tech oriented boat. In particular, the site you mentioned says "We provide dual steel 12 litre 300 bar cylinders", I'm sure they happily provide singles as well. Now diving manifold doubles on a rec. oriented boat and having them provide them would likely be harder. Independent backmount doubles is a way to use singles that a boat has. Or just dive a single to practice backmount tech skills, sans valve drills.
Obviously sidemount allows you to use whatever single tanks they have, but depends on the boat being happy with sidemount, and you doing it smoothly. I went sidemount partly for the travel redundancy it offered.
I echo the view that you need to dive locally, even if it is a weekend trip for just 3 dives. And/Or find some pool that has some scuba club practicing. You need the water time. You can learn/practice a lot in the pool. DSMB - Pool. Buoyancy/Trim/Propulsion - Pool. Sidemount tank handling - Pool. Or small 30' rec dive. Having a DM at a shop with a pool give you a 'refresher' in their pool every other week may be cheaper than travel. And involve less gear cleanup.
I think you need more experience of tech, side or back. It seems you could do that locally on weekends. Then you'll have a better idea.
RainPilot's plan is a good one. And jgttrey's notion to get to rec-trimix first, which is essentially half of RainPilot's step one. (I'm just a BP/W diver new to sidemount that wandered into this conversation because I can spell deep dive and CCR.)