I think a toast is a very appropriate, lovely and touching idea, but it matters most what your mom thinks. Once you're at the venue, and it's no longer a surprise, ask your mom how she feels about a toast to your dad. She may love the idea.
After losing a spouse, many family and friends shy away from talking about the deceased because they don't want to upset the widow/widower. I can tell you from personal experience that this is very sad. When this happens, it feels as if our loved one never existed. On the contrary, it is so wonderful to hear someone else say our loved one's name out loud.
You can't hurt a widow/widower by bringing it up. They live with the absence every day. Six years has passed, and your mom is probably past the very raw grief part. She has likely incorporated your father's absence into her daily life, but that does not mean she doesn't miss him like crazy, doesn't wish for all the world that he were there to celebrate with her. A small toast won't "steal any thunder." She and your dad shared nearly an entire lifetime together. I think a toast is a lovely gesture to acknowledge that lifetime.
As for "depressing" the celebration? Everyone already knows your father is deceased. They are treasured family and friends. I would give them the benefit of the doubt and assume that they have the emotional strength to handle a toast that will probably last less than a minute.
Whatever your mom and you decide, I hope she has a wonderful celebration and that she truly feels all the love that you've put into this special day for her.