The way to change respawn wave times is to send an input to the tf_gamerules entity. The input needs to be either SetRedTeamRespawnWaveTime <float> or SetBlueTeamRespawnWaveTime <float>. This number will then be the time between each wave.
When a player dies, they must wait the time of the deathcam, then wait out until the end of the next full wave. For example, if the respawn wave time is 10 seconds, a player who just died will wait out the deathcam time which is a few seconds, then wait between 10 and 20 seconds depending on how far through the current wave is.