Worry, meaning book a needle test this fortnight, when a lump is growing, appeared suddenly, changes size day to day, is ulcerated or irritated, sits on a lower leg or the face, or when your dog is bothered by it. A soft, stable, slow lump on the body of an older dog can reasonably wait for the next scheduled visit, but it still gets tested then.
The rule that beats all guessing: lumps get named, not watched on vibes. A fine needle aspirate costs a few hundred dollars and turns a mystery into a plan; experienced vets guessing by feel are wrong often enough that guessing is not a plan. Mast cell tumours in particular are shape shifters that can look exactly like harmless fatty lumps.
Keep a lump map: photograph each lump next to a coin, note the date, and you will never lose track of which one is new or bigger. It is the single most useful thing an owner of a lumpy older dog can do.
Send what your vet said and quoted. A veterinary surgeon reads it and comes back to you with what they can do and what it would cost, before you pay anything. A written second opinion is from $250.
Send the case, hear back from a surgeon with what they can do and what it costs, and only pay if you go ahead.