Hi Biggles, and let me be the first to welcome you to the Org! Trust me, your thread isn't long... you should look at some of mine ;-)
Okay, firstly. The Julii full-stacks. You will have problems beating their legionary fullstacks, that is true. I will advocate plentiful bribing to remove those armies you can't fight. Assuming that by Marian times they will have 7-8 provinces (assuming, because no Roman faction has ever survived till the reforms before in any campaign of mine) it will be easy to take down their peripheral provinces to drive them into debt. If they don't, then their home provinces are there are, and if you defeat enough of their armies they will run out of population to raise any more new armies. That happened for me as the Julii against the Gauls. They kept sending 20-stacks, then slowly it dwindled to 15-stacks, then to 5-6 stacks that I just irritatedly bribed away. So it will be for the Julii. Bribe. If you've conquered so many places you must be making at least 20k a turn with full economic infrastructures up and running. Denarii were earned to be spent, so make them well-spent. Save your game before bribing any family member of the enemy, because once you fail he will never be bribable again. If you can't bribe him, then the price is simply not high enough. Save up more. You must be patient with the Julii. Don't try to make gains; just sit back and make Mediolanum-Patavium your frontier, building forts at every Po river crossing. The AI never sieges forts, so your frontier will be safe while you subtly undermine their capability for war. Meanwhile, expand elsewhere to get more money for ever more ambitious bribes. Don't fight the battles you can't win. To quote, 'Infinite gold are the sinews of war.'
If you really want to fight the Romans, lure them into bridge battles whenever possible. I recommend the one just west of Rome. For that, though, you will have to undertake a risk in transporting a full-stack army by sea behind the Julii lines. As long as you stand -on- the ford itself and allow an army to attack you, you will have a considerable advantage in merely defending the ford against any comers. When fighting that battle, simply mass your troops in a semicircle on the other end of the ford. If you cannot fight at a ford, then your battles are sure to be high-cost, but still winnable. In this case you would keep your chariots in the rear, all massed at one spot on the line. You must take the offensive even in a defensive battle if your troops are to stand up to the enemy. Stretch your line to match the length of the Roman line, and engage them along the entire length. Then, when all are engaged, hurl your chariots at weakest point in the enemy line, charging right through the enemy. Since Romans are sword infantry they will not have any bonus against cavalry/chariotry, so you can burst through them. Chariots can frighten infantry, so use that effect to the max. Charge and charge again. If you have light chariots, rain arrows. If you have heavies, then use them for their god-given purpose. After one unit routs, devote your tender loving care to the ones next to them. The more units rout in a short time, the better since the higher the chance the rest of the army will rout. Your chariots will take great maulings, but they are your decisive arm and since in one-to-one combat you cannot beat legionnaires, the only way to kill them is to make them run. After a victory, retreat and retrain your chariots. Then return for more. After you destroy a few of their stacks and you can see there are only one or two left, begin the besieging of their settlements. Keep using bribe if you can to make your life easier. Remember your objective isn't to win every battle, but to capture provinces. So if you can avoid a fight, well and good.
My strategy against the Romans is, despite all that typing, only theoretical, because as I said, I have never faced legionnaires except in the Roman civil war. Otherwise, I make Rome my first priority for conquest so they cannot get Marian reforms. Furthermore I have never played Britannia before, but I have fought them in many campaigns and squared up against Romans as Gaul, which is pretty close I daresay.
Secondly, Large Boats, buildable at shipwrights. They are your only answer (if an inadequate one) to Triremes. Don't get yourself into a naval battle if you can help it. Build only what ships you need to accomplish your transport objectives. That is the lot of a barbarian navy. They are simply -not meant- to engage civilised fleets. Not at such a late stage when they have good ships. Alternatively, if you -really- want to sink a few ships, use multiple fleets. 3 20-stack fleets of large boats will certainly be enough to destroy one 20-stack fleet of triremes. To ensure the enemy does not live to fight another day after his defeat, place each of your three fleets on one of the 8 red squares surrounding the enemy fleet, then put one ship into each remaining square. Being completely surrounded, an enemy fleet once defeated will sink and never surface again. That's the only naval trick you can use without naval battles being played out.
Thirdly, I think you have the wrong temples. Try the one for the god of war or something to that effect. Reading the long description of the temple will give you some good clue to what they can build, since it is not apparent from the description of shrines at the beginning. Alternatively, just google 'RTW + temples list' or something to that effect, and download whatever spreadsheets and PDFs come your way. I have one that is really very good--tells you all the traits, effects, bonuses, and buildable units come with each temple all on one excel spreadsheet which is a little hard on the eye at font size 7 but tells you everything.
Fourthly, there is no such thing as Bastarnae cavalry. I assume you mean Sarmatian cavalry? That's the only heavy cavalry mercenary unit I know of. Sarmatians are good starter troops for shock cavalry-poor factions like Thrace and the Scythians. But after you develop your own cavalry capability they will almost surely be outclassed by your own buildable models. Bastarnae infantry are good mainly because of their 2HP, and I regularly use them in tandem with their brother mercenaries the Thracians, as general-hunting forces. I attract the general to charge, then position them in depth right astride the enemy general's path. They can cut a bodyguard cavalry into pieces in no time.
Hope this has been useful, cheers! Take two bridge battles and see me in the morning. ;-)