It's not too late.

The PB would have to expose a small portion of the existing rebar under the spa by chipping away the concrete in order to attach a bond lug for a bond wire to meet the electrical code requirement. That bond lug and wire would have been installed and inspected prior to the gunte shoot.

A number of holes would be drilled in the spa face generally 12" apart. Pre-cut reinforcement bar would be placed in these holes which are filled with epoxy to secure them. The location would represent the size and shape of the bench. The prepped bond wire on the existing rebar, will now be attached to the new rebar.

The rebar cage would be produced just like your original work on the pool, and then the bench is shot and shaped.

The surface must be wet but not soaking, to allow for proper adhesion of the new gunite.

Allow 28 days for the new bench to fully cure.

We have done this method for years on older concrete pools whereas some clients wanted to add benches or steps.

As far keeping it clean, it's inevitable. Your top steps in your pool will collect some debris too. It's not a big deal to just sweep it off.

We install in floor cleaning heads on large sun shelves to push debris to the pool floor as the auto cleaners cannot get to that area. Treads on steps will also get the cleaning heads.