Suggestions for where to buy a dress!

My cousins wedding will be in early October in Maryland, before the hate starts pouring in even if the Coronavirus is still a threat they will hold a small ceremony and party that we would attend since I am one of two cousins he has (less than 30ppl) but let’s not focus on plan b. Their wedding colors are purple and gray, and they have requested guests wear any shade of these colors, a traditional Niaja (Nigerian) dress or fun cocktail attire. I am one of like 3 women on the grooms side and want to respect their wishes of wearing purple and am scouring the internet for purple dress. Thought it would be easier in prom and Easter dress season especially for a purple/lilac/lavender tone but everything is ether too skimpy and meant for a 18yo or very plain. At 29yo I have a decent shape but wouldn’t go for skin tight since my tummy isn’t the flattest...thanks mom pouch...I want something beautiful in that color wheel. I found one dress at kohl’s and it arrives tmmrw but I kinda already know it’s a no-go, I was going to plan on adding a belt but didn’t realized how low the back was and don’t want the belt on my bare skin cuz well I like to party and dance and it be a sweaty sticky impression of a mess before 10pm. And no belt isn’t an option I needed it to give me a shape otherwise I look like I’d have a bag on.

Front view

Back view is only avb in orange but you get the idea...

So any suggestions on maybe modifying this dress? Is it too summery for a fall wedding? We will be walking from/to the hotel and venue a total of 4 blocks we could always get an Uber if it was really chilly but it’s 4 blocks 🙄 should I just keep the dress for the summer and keep shopping? ( sorta feeling this way) what are some good places to buy ‘cocktail’ dresses and I’d like it at or below the knee. Already went through alllllll of amazon, kohl’s, Macy’s, jcpenny, local mall...any suggestions of where to look will be greatly appreciated!!!