I don't think it would be worth it to try for a genuine full-scale invasion just now. To build a retro fleet which is competitive with what our opponents currently have in the field (and for it to reach the battlezone) will take quite some time and ressources, which IMO are better spent to stand up our two radiated fast and speed up reasearch at our artifact world. I would guess that any time we might save upfront, we will lose later by delaying Impulse even further and having the construction of our impulse fleet slowed down by the upkeep for the soon to be obsolete retro fleet.
Maybe we should consider making a play for Xengara and Sol instead, the spuds in the east, where we might be able to sucessfully snap in a colship with moderate escort (and heavy diplo use) at some point as you and dathon managed to pull off with Kronos. Of course, the chances for such a stunt to succeed a second time are slim, OTOH such an operation would be cheap and can be attempted with our existing fleet over Kronos which is no longer needed, now that the planet is fortified (plus a colship and maybe a single bomber to wipe out baby colonies). Its job would mainly be to shoot down colships and retreat & return, letting the AIs do the real fighting until an opportunity opens and then to "defend" the new colony with alliances and threats.
Another reasonably cheap option would be a limited punitive operation against Altair as we are already at war with the birds. They have no PS, no ECM and only Hyper-X ATM and are at war with anyone else. Our existing MBs should be effective against their bases - add a handfull of bombers (preferably using two different slots for decoy effect) and we have at least a shot at getting the planet, and if not, it will still boost our relations with anyone else.
As both tactics would probably involve repeated hit&retreats with MBs, there are also ethical issues to consider. And I still think that, all in all, no matter what we end up doing, it won't make more than a couple of years difference for a conquest victory, so the only real benefit would be to make the game more interesting.
ignatius