Hello, this is Dr. Benedetta, and I would be glad to help.
This is a tough situation, and I feel for you and Todd.
Amputation can be a good solution, and 3 legged dogs can do remarkably well, with a great QOL.
However, if the tumor comes back as a malignant aggressive tumor, I would run all the other tests, such as chest radiographs, abdominal ultrasound, blood work, to assess whether there are is metastatic disease, before moving forward with surgery.
Also, I would take into consideration Todd's general health. If he is healthy, and the lump is his only problem, then he is likely to do well after surgery. If he is overweight/obese, or with a known condition, such as diabetes, liver/kidney issues etc., the surgery can be tough.
Finally, I would also consider how he recovered from his recent surgeries. If he bounced right back, great! If he struggled, or had complications, then a major surgery like an amputation may be difficult for him.
Hope this helps, let me know if I can be of further help!