Finding chair and boat transition states during ring-forming reactions
As you have figured out, this is a non-trivial problem.

It's okay to find a boat conformation that might be a bit higher in energy and see if it can map to a TS. Keep in mind that for 6-membered rings, there are many conformations beyond chair and boat. A half-chair, for example, might be a useful test.

In general, for 6-membered rings, using ring-puckering searches are useful. Calculate the plane of best-fit for the 6-membered ring and displace atoms up or down -- this is similar to sampling the out-of-plane vibrational modes of the ring.

So the ideal situation for the TS would be to consider a set of conformations, that might be higher in energy than the reactant and product. After all, it's a transition state. Then use a nudged elastic band or similar method (your bond-lengthening) to look for the real TS.

Good luck!