Hi, I have a BES820 - the model down without the built-in grinder.
10 seconds points to too coarse a grind and/or too light a tamp.
Does it say "over-extracted" or is this just the "red" area? My pressure guage doesnt have words or numbers on it, but I would go with the feel rather than the words. Try and grind finer / tamp harder until it gets closer to 20s (30s might be a stretch for this machine). I find this is just into the red on my guage.
Also, the bitterness might be coming from the grinder - I always find the Breville standalone conical burr grinder makes inconsistent sizes, meaning some could be getting burnt and bitter. Just how fresh are the beans and how are they stored (by the retailer and by you?)
And finally give it a good amount of time to warm up, and pre-heat the basket and group.
Just some thoughts - others will no doubt chime in :)