Since I'm a nomad player too, I'd like to share my 20 millibucks.
My current gross profit is about 90000/100000/90000 (35000/45000/35000 from trade), but my technological backwardness is very remarkable. It happend because I was in the production, not trade and diplomacy. Really, it's the only way to survive - to offer your services. Maint. free and fast ships, zero-gravity BSYs, some useful gadgets (I've picked a religious trait to get War and Death Talisman).
In the end you'll be probably obliterated by the ships you've sold to your former ally, but it's not a problem, because you can hardly expect to win such game as a nomad.
I've tried to gather some notes for adamantic nomad player, I hope they'll help.
During race creation: minimise your resource mining apptitudes, cunning and possibly intelligence (space born labs are not affected by your racial research setup), max your agressiveness, defensiveness and conctruction apptitude (110/110/150). Political Savvy doesn't matter, since you only can get 15% (10% cap * 150% max) from trade. Pick all usefull racial traits (except heavy industrual), pick Naturalist trait for huge amount of free points (even if you'll manage to get several colonyship, you won't be able to build any reasource/research/intel ganerating facilities).
In start build only BSYs and remote miners(sat or ships). Expansion is not very important (except if you have home system withou asteroid belt) and 2-3 scouts will explore several nearby systems easily. Build SYs in geometric progression but wisely, because you're spending you initial capital and have a miserable income from mining. In research go into stellar harnessing, ship construction, cargo, propulsion, mines and warheads. As soon as you get ship hull of 220-250 kt build dozens of such ships with small SY and recycler nodes onboard and send them to the outer systems with asteroid fields. Such small shipyard should be able to construct minimal BSY during the 1 year in emergency mode. After it's finished - scrap shipyard, build another one and send it to an empty system. Then start forging BSYs and storage bases (in case you use sats for mining).
Supplies are a huge problem for nomads - they usually nave no planets to resupply. So build transpot ships with a small amout of movement which are filled with reactors and solar collectors. Hide them in storms or nebulas. Also trade, do diplomacy all this time (I was not very good at it, but try to improve).
Such economy is very fragile - either someone will destroy your mining stations or he will start to convert asteroids to planets. I guess it will be the end of empire unless you have ovewhelming forces to protect yourself. But you had fun
