It would help if I reread posts to refresh my memory! You don't need to worry about CYA. You probably don't have any, unless you used Dichlor to shock/reactivate the bromine (usually you use MPS or bleach, but some people shock with Dichlor in which case you'd build up CYA).
Yes, the borates could be added by a combination of 20 Mule Team Borax plus acid (the Borax by itself raises pH), BUT it's much easier just to get Gentle Spa since it's pH balanced (it's mostly boric acid). It's more expensive this way, but it's not a large quantity for a spa.
In your 450 gallon spa, if you use Gentle Spa, then it's around 2 cups 3 ounces to get to 50 ppm. If you use Borax and acid, it's around 3 cups 1 ounce of 20 Mule Team Borax and 1 cup 3 ounces of dry acid (sodium bisulfate). However, I would add portions of each alternately so as not to make the pH swing too high (or you could mix these in a bucket of spa water before adding it back in). You can also use The Pool Calculator to figure dosing.
Hmmm not sure I have that much dry acid left now - Need to go order some 'Gentle Spa' I guess!!
Checked tonight and pH is now 7.8 again so on the way back up.... What is also strange is I checked with test strips and these (aqucheck bromine strips) seem to indicate the pH is low 6.8 or so) - The Taylor test kit (new reagent) shows 7.8.... So now I'm not even sure I trust the results I'm seeing!