Trying to understand adult dog before deciding to get a puppy

My wife and I lost our sweet Noodle doodle at the end of October unexpectedly to cancer and his younger sister, Maple, a 4 year-old 90lb Old English Sheepdog has become even more of a couch potato that she was before. My wife and I are starting to feel ready to have another male dog in the house and are considering bringing in a puppy but have some concerns with Maple regarding if it would be appropriate to bring in a puppy with her.

Maple always loved Noodle and has always been protective of him and our cat. Her being the youngest of the animals, she slotted in easily to the household. Unfortunately she was kicked out of doggy daycare as a puppy because they separated her from Noodle, she got anxious, and tried to herd the other dogs several times. They never told us about any of this until they told us not to bring her back. This was the only daycare in the area and we live in a fairly rural area so not a huge amount of other options for getting her together with other dogs.

Maple is very sweet with people, slightly guarded at first but warms fairly quickly. When we have had her at dog parks however, she instantly became protective of Noodle on the many times we tried getting both dogs to one, barking at and lunging at other dogs. When we tried taking her on her own however, she would occasionally do the same. She has also snipped at my sister's dog when they first met when that dog tried to say hello, although that was perhaps not the most controlled introduction as I was holding and petting Maple trying to keep her calm. She also had a history of snipping at Noodle very occasionally but was always solved almost instantly between them. With other dogs the interactions were never allowed to go further.

So with that in mind, I'm looking for some insight on how to know if it would be OK to bring a puppy into the house. We are obviously 100% on board with the standard protocol of gradual introduction, crating, training techniques, etc as we want to have a successful relationship at all angles. We also don't want to have missed some glaring red flag and have scarred another dog who might not be compatible with our current dog. I cannot handle the thought of that. I will add, it can be hard to read some of Maple's body language as her tail is docked due to stupid AKC standards.

Please ask any questions for clarity. My wife and I truly only want what is best for the dogs and are unwilling to barge into a situation with a puppy just because we want a puppy.