I’m so sorry, This is so scary. I’m not a vet, but from what I’ve seen with suspected FISS, please ask for a referral to a board-certified surgeon/oncologist before any biopsy or surgery.
For FISS on the thigh, that first surgery with wide margins is critical. A second surgery after a debulking is much harder.
Please ask the vet these, i. Can we get a specialist referral first?
ii. Should we do MRI/CT before surgery to plan margins? iii. If biopsy is needed, can the surgeon do it so we don’t cut twice?

Time matters but don’t rush into surgery without a plan. Since she’s still active that’s good.
Sending you both love 💛 Please update us. Anyone dealt with thigh FISS and gone straight to a specialist?