A limp at six months is outside normal TPLO recovery and deserves investigation, not reassurance. The common culprits, roughly in order: a torn meniscus that was intact at surgery and has gone since, implant irritation or low grade infection, arthritis already established in the joint, or the other knee quietly failing and unloading onto the operated leg.
The workup is straightforward: examination, comparison x-rays against the post op films, and sometimes joint fluid analysis. Each cause has a different fix, from a small second procedure for the meniscus to implant removal for irritation, so naming the cause matters more than any general advice.
Start with the operating clinic; they know the knee and most stand behind their work. If their answer is a shrug or an open ended wait and see, an independent review of the before and after imaging is exactly what second opinions are for, and post operative cases like this are among the most common we see.
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.