That's a tricky situation -- ideally you'd scout first and then unlock in response to what you find on the other side, so not having free knowledge is a mite of a disadvantage. But! All is definitely not lost.
As the LAST resort, you can always nuke the planet. So if all else fails, that won't for this -- but the price is steep in terms of AIP, so it's not something to do lightly. As another near-last resort, you could use EMPs, but that's also not something to do too lightly.
You can also flood the system with many transports (as a general rule) to increase the survivability of them. So send in 5 or 10 transports instead of 1, and they'll live 5 or 10 times as long. Still that probably wouldn't do it here, but I figured I'd mention it.
Bombards are sub-sniper range so the scout starships won't help you.
However, if I recall bombards now really stink against force fields, so you could bring in a force field of some sort with you. If you have riot starships, you could do it via those. Or if you don't, I don't think there are many other good options force-field-wise in the very short term unless you have spire shield bearers as an option via asteroid mining (having spirecraft turned on, which are off by default because they make the game way harder). You could build a force field on that planet, though, if you can get a mobile builder and some engineers in there, pause the game, and get them all building the FF as fast as possible. There's a good chance of them getting killed too fast, though, so you may need a diversion with your main fleet to get that established. Once the FF is up, then it can move (painfully slowly) around that planet to protect your fleet as it moves. Not the first option, but it would work.
Another thing to bear in mind in general is the divide and conquer trick: make sure that as little as possible is shooting at you at any one time. To do that, check out the ranges of your enemy with the Z+X hotkey. Go where the fewest circles overlap, and start whittling. It might take multiple waves, but that usually can give very good results even with bombards. Unless your situation is just abnormally unlucky (which does happen, but that's why scouting ahead also is critical with bottlenecks in particular).
Another option, if you're playing with the Zenith Remnant expansion, would have been cloaker starships. However, those cost knowledge so you're kind of trapped out of those for now. But they keep your ships in a bunch invisible, and thus keep the enemy from shooting you.
Lastly, something that is generally effective against very long-range ships are ships with radar dampening. Specifically, raid starships are a good example. You could build a full complement of those, put them in a transport, and bum-rush the AI command station, the AI guard posts, or otherwise to try to cripple the planet. Meanwhile have your forces (plus turrets and minefields if need be) waiting on your own side of that wormhole, on your planet. If you can get the guard posts killed, the bombards will likely move closer to your wormhole so that you can pop through and surprise them without getting killed before being in range.
I can't remember offhand how much armor piercing bombards have, but if they don't have much then you also want ships with a lot of armor. If you can look at the stats and see what you have that is strongest against bombards, then send in transports of that type of ship NOT to strike the bombards, but to strike the guard posts. Send in lots of fighters as chaff for the bombards to shoot at, if your stuff is still dying too fast.
And... off the top of my head that's all I can think of. It's definitely a predicament you're in, but it's not a permanent roadblock. There's always the nuclear option if all else fails. But your AIP can only absorb so many of those, so use them with care!
Good luck!
