2010 - won't start - dash lights up like Christmas tree

Looking for advice on this strange problem...
2010 Prius.
approx 75000 miles.

Went out yesterday, and car would not start, and dashboard lit up like crazy and started doing odd things.
1) Car did sit for approx 5 days without being used (today is day 6).
2) we had 1-2 days of warm temps (high 50's) and 2-3 days of cold temps (28º or so).

in 2015, I let car sit for 7 days and battery died completely. Jumped it and car was fine ever since.

I did a lot of searching on these message boards and my gut is telling me that it is a 12v battery issue, but wanted to hear what some of you thought about it.

Press Brake, push on button like normal, dash lights up.
I hear beeping noise 5x.
various dash lights are lit:
• Slippery when wet
• seat belt (I forgot to check if I had seatbelt on at that point, so that might go out).
• Battery light
• Brake light (emergency break) - (E brake is off).
• Check engine
• circle that looks like a steering wheel with exclamation point
• ABS
• Low tire pressure light

Now here's some more weirdness...
Radio appears to be on (I have Sirius and it tells me what song is playing) BUT there's no music.
When I press the break pedal, the radio turns off, and display goes blank. When I take foot off brake, radio turns back on (again with no sound).
As I sit and watch the dash, sometimes it goes off completely, sometimes various lights go out.
I hear a blower running - which I think is my heater.

Headlights seem to be ok (maybe a little dim? Checked it during the day).
Dome light seems to be ok - again might be a little dim?

Could not shut the vehicle off, with foot on break, had to remove foot, then hit ON button and it seems to shut off then (as if it were in AUX mode).

I tried using a jump battery to jump it using the front fuse box method, with no change - same symptoms.
(jump pack only registered at 50% charge, but light said it was ok to jump).

Tow truck is already on it's way, and will be dropping it off at dealer in a bit, but wanted to get some of your thoughts before I hear from the dealer - which they said their booked solid for today and won't look at it until tomorrow morning.

Sorry so long winded.

 

Thanks for the replies already.
UPDATE: Triple A guy showed up, asked what the issue was (I explained the above).
He got in the driver seat, saw the same thing, and looked at the dome light, and said without any hesitation, "It's your battery."
He proceeded to get out his "JNC660" portable jump starter, and hooked it up via the front fuse box, and sure enough... it started up with no problems at all.
I've since driven it around for about 20 min, and it has restarted 2x now with no problems.
I won't cancel my Toyota appointment until I get up early tomorrow and test it out after it has sat overnight.
I may still need a battery, but for now I'm very much relieved that at least I know that's what the issue was and what caused all those crazy lights to come on.

I'll post again if there's any further change, but my lesson learned... don't let the vehicle sit for more than 3 days without running it.
I'll start saving now for a new battery.

PS> Thanks to earlier posts I saw here, I called 2 local toyota dealerships and one quoted me a price of $298 for a new battery (labor inc), and the other quoted me at $200 (with Labor). BIG difference, and I'm glad I called ahead.