Since I am mostly playing multiplayer (usually cutthroat non-roleplay multiplayer) so I may have some helpful advice. A lot of it may sound opiniated and weird from singleplayer perspective and feel free to not follow any of it, it's just a synopsys of my experiences. I don't know how "veteran" are these veteran players and how aggressive they can be. While to a newcomer to MP attacking a neighbour in 2230 may sound early, it's not infrequent to be conquered as soon as 2208, my advice would assume they are cutthroat even by my standards.
First of all, the best way to learn how to not die to early rush is trying to make an early rush yourself (you can test it vs AI), that way you can see the weak spots and what do you need to expect. Start a new game with race with industrious+agrarian, switch everything into militarized economy (do not use nutritional plentitude), build a mineral district and as many alloy foundries as you can (in many cases it is advisable to override lab/artisan building to fit an extra alloy foundry), build at most one outpost (to save alloys). Your first tradition should be supremacy. Build few more science ships to scout neighbours. Then at some point start producing ships from two shipyards (all your starbases should be either shipyards or anchorages), build 4-6 troops, claim your neighbour's capital and invade it. This way you should be able to have 20-30 ships in 2208 (depending how optimally do you do everything) or around 40 ships in 2212. This is what you should be prepared against, most people aren't going to do fully dedicated rush but it's handy to know that it exists and what is the fleetsize you need to be prepared against.
To defend against early aggression first of all you need to figure out if it's coming. Explore your neighbours, if you see he has too few outposts, he is saving alloys. If you see they have less colonies than you either they are bad or they are are saving alloys. Offer a non-aggression pact, if they refuse they likely want to kill you (even if they make a "I don't want to waste influence" excuse). If you offer a trade deal you can check their alloy stockpile/income in diplomacy screen, if you see it's way larger than yours, you might be a target, start spamming foundries everywhere and try to catch up. Keep checking their alloy stockpile once every 2 months to know when did they order their fleet and prepare yours when you see that they suddenly managed to spend 3000 alloys in one month.
In an early game war invader cares only about your pops, they wouldn't bother conquering or even claiming worthless planetless systems, their main battle prize is your capital, so you have to fortify it most of all. Replace your trademodule with gun station (in many cases it's better to replace both modules with gun station and have shipyard elsewhere) and position your fleet behind the starbase, so if enemy wants to attack your fleet they will have to engage starbase first. When starbase's shields are almost depleted, engage with your fleet. If you do everything well you should be able to defeat 40-corvette fleet with 25-30 corvette fleet so you have a defensive advantage. Still, you need to have defensive fleet, gun stations alone won't cut it if they dedicate to killing you.
Chokepoints may help vs ai but a smart human will just fly around your starbase, so there is little point in creating a fortified starbase outside of your capital. One more weird thing, frequently people offer you to become their tributary/subsidiary. If it happens, check their military power/alloy count before opening the diplomatic message. If it looks as your enemy has bigger fleet I heavily advise you to just accept the offer. Sure, taxes will hurt your economy a lot, but if you refuse they are going to claim your capital, then declare tributary war. After you lose it you will not only become a tributary (which will cause you to pay taxes) you will also lose your capital which will probably contribute to being 80%+ of your economy. On the other hand if you accept you can later engage in a war together with your sovereign and by doing a timely betrayal you can come back. Also I heavily advise you to talk to people, if you see an aggressive warmonger nearby you can often be more useful as an ally, for example you can talk them into exporting alloys to you in exchange of energy/consumer goods (remember, they will have militarized economy so it can be profitable for them).
Once early game rush is dealt with, you should procede playing as normal, economy development isn't that different between multiplayer and singleplayer (although to be fair a lot of people are going to mismanage it), one thing which can sound weird for many is that Megacorps are one of the best (probably the best) empire for early game rushing and for defending against rush. They start with 4 more pops than normal empries making their economy better (don't forget to replace commercial zones with something useful) and private colony ships allow them to expand while saving precious alloys, so you are able to both be safe against enemy and be growing at a steady pace. Main advice I would give you is to have 0 clerks throughout the whole game. Also for a while it's better to produce food and sell it to get energy rather than produce energy itself. One surpsingly useful MP civic is technocracy. If you are playing with cutthroat players, everyone should be in a constant sense of fear and be afraid of dyign at all stages of the game, that makes spamming labs highly punishable, if you see there is a neighbour which has no alloys and is probably spamming labs he is an easy food and once he is dead all those labs will be yours. Because of that, alloy-lab ratios are higher than in singleplayer, so bonus from science director ruler job contributes to a lot more. One of my favourite strategies in MP is to play technocracy, don't die in early game and try to rush for fastest neutron launchers (neutron launchers are the strongest lategame weapon in the game from cutthroat MP perspective, a person who gets them first has a huge advantage over opposition).
P.S. I head you saying smth about terraforming, I m pretty sure that it's not that important, it's better to colonise planets even if they have low habitation (especially if you are playign agrarian-industrious race which I highly suggest) rather than wait for an eternity before you have terraform techs and energy.
