Then that's precisely what went wrong. Bummer
Even if you hadn't pre-washed the shirts to get out sizing, you still would have gotten a better result.
Pool supply stores and places like home depot carry soda ash, called Ph-UP. Though it may be more than you want. A fine art supply store, not joans etc, will probably carry smaller packets of soda ash.
24 hours is a good amount of time to let the dye react in the minimum temp range, roughly 76'. However, every 18'F increase in temp. cuts the reaction time by a third.
As far as washing out your dyed garments, I toss them all together in the wash without any pre-rinsing. I wash it in hot water with my detergent. Usually synthaprol but I have used regular liquid detergent before without issues. I run the washer through the first wash cycle, let it dump and then reset it back to the wash cycle before the rinse cycle starts.
Don't get discouraged. Ooo..if you do have a fine art supply store nearby, they probably also carry quality fiber reactive dye. Probably Jacquards. Pearl Arts also carries them. They have everything!