Your post has left me with lots of questions:
Why must your current system be used outdoors? If it's just because of the hose adapter, you can either get
one that will adapt the current fitting to an indoor sink faucet, or you can connect it to an existing pipe using a
saddle valve. That's what I have done with my counter-top RO sustem. Turn it on to produce pure water, off again when done.
I'd urge you to look more closely at that "under-sink , tankless" system. A 100 gallon-per-day system only generates about a cup per minute, so some sort of water storage must be used. Most residential systems come with a 3- or 4-gallon bladder tank.
The tankless waterdrop systems are 400 gpd systems, so are "on demand", but they use proprietary filters and membranes, so you're always beholding to them for replacements, which are fairly pricey. The use of standard components is a much safer idea, as if the supplier is out-of-stock or goes out of business, you can still get replacements from any number of places.