ITBs look cool but such will reduce tuning options. You will find them to be really hard to tune esp without a plenum feeding them.
The L5 ef-el piston is a dome shape. It has poor squish. Upon spark the combustion flame for most available fuels has a long way to travel before the piston gets pushed down. When these pistons are pushed for power the limit is found quickly and it is typical that they get to a point of needing huge timing advance to get the flame burning and give it time to travel around the dome before making max pressure to push down. Now a Mazda Skyactive has a dome but it has remarkable squish with the majority of the dome coming within a fraction of a mm of the combustion chamber. But it works because they put a small pocket in the to where combustion occurs and the spark is fired really close to TDC. The EF-VE is basically a flat top piston. For NA this is the way to go. In looking into this I was going in the direction of using a motorcycle piston from a GSXR 600 as the are strong, forged, light and have thin rings and thus can be revved and have little friction so “more power”. Plus they are short so you can use a long rod for a good stroke to piston ration in the EJ-DE block I was going to use. Alas life got in the way and it never came to fruition.
There are no variable compression Daihatsu motors that I know of, only DVVT or variable cam timing.
“Snappy” will need high compression or a way of getting the turbo spinning at low revs. My experience is that a high level of spark and fuel control along with a closed loop feedback system is critical to controlling and maximizing what is happening. So #1. the electrics and a sophisticated ECU with data logging for the tuner will be essential. #2 choose a turbo for response. I’d be trying a RHV4-VJ38 as they are a common variable vane turbo that is not stretched until over 180hp stock or move with billet inducer and some other mods. These are a common diesel turbo where low down grunt is needed and I’ve been able to get excellent boost from them just off idle.
There are plenty of direction you could go, but above is a few quick thoughts.