Full time professionalism in the second tier is a pipe dream, it'd take years of hard fought growth for that to ever be feasible.A full-time NRL Division 2 (without promotion & relegation) would help here. It would be a fairly decent lineup too:
New Zealand 2
Sunshine Coast Falcons
North Brisbane Devils
East Brisbane Tigers
North Sydney Bears
Adelaide (or a slot for them if / when any bidders want to bid to enter a team)
With the international flavour, relatively wide footprint in Australia and the mix of traditional and new clubs it could be a very popular comp that would attract sponsors and rate well enough on TV to get at least an NBL level TV deal to allow it to be a full-time comp that is also affordable for these clubs to play in at the same time (players salaries will be much lower than the NRL).
The NRL then have a pool of potential active clubs to choose from when they want to expand and if they need to make room for that expansion, they have a viable pro comp to demote teams into that won't destroy them.
Example 1: although Manly would prefer to play in the top flight, being demoted to this comp, remaining a full-time outfit and rekindling their rivalry with the bears in that context would be much more palatable than being demoted to the NSW Cup as it currently stands.
Example 2: if the Gold Coast Titans can't manage to turn things around, the NRL have both Burleigh or Tweed operating as pro clubs in NRL 2 that can step in and take the licence.
Clubs like WA, Adelaide, Easts Tigers, Redcliffe, Sunshine Coast, Central Coast and Ipswich (or any other club with top flight ambitions like North Sydney) can all prove their ability to run pro clubs, build ratings, attendances and memberships to push their case for inclusion in the top flight.
Also if it's going to be feasible chances are that international teams aren't going to be part of it, at least not initially. That's not a bad thing either, as the NRL should be incentivising most of these other nations to invest in their own projects and to diversify their income sources independent of Australia, not making them more an more reliant on the NRL and Australian system for their existence.
However if the NRL is insistent on making them all dependants, then they should totally take them over so they can more directly benefit from it, not string them along as they do now, but I'd argue against that, and this is a separate argument.