After the time I came up to the working solution. I decided to use Celery task to calculate next occurrence date, when training passed.

Future improvements can cover celery-beat schedule, to make sure all trainings was updated at the end of the day.

I can't pretend that it is the best solution, but if anyone has more experience working with this type of work, I would appreciate any changes or improvements.

models.py

class Training(TimestampModel):
    template = models.ForeignKey(TrainingTemplate, on_delete=models.CASCADE, related_name='trainings')
    start_time = models.TimeField()
    end_time = models.TimeField()
    location = models.ForeignKey(TrainingLocation, on_delete=models.PROTECT, related_name='trainings')
    recurrences = RecurrenceField()
    next_occurrence = models.DateTimeField(null=True, editable=False)
    next_occurrence_celery_task_id = models.UUIDField(null=True, editable=False)
    members = models.ManyToManyField(Member, through=MemberTraining, related_name='trainings', blank=True)

    objects = TrainingManager()

    class Meta:
        ordering = ['next_occurrence', 'start_time']
        indexes = [
            models.Index(fields=['next_occurrence', 'start_time']),
        ]

    def save(self, *args, **kwargs):
        self.next_occurrence = self.calculate_next_occurrence()
        super().save(*args, **kwargs)

    def calculate_next_occurrence(self) -> datetime.datetime | None:
        try:
            now = timezone.now()
            today = timezone.make_naive(now).replace(hour=0, minute=0, second=0, microsecond=0)

            next_date = self.recurrences.after(today, inc=True)
            combined = datetime.datetime.combine(next_date.date(), self.start_time)
            aware_dt = timezone.make_aware(combined)

            if aware_dt > now:
                return aware_dt

            next_date = self.recurrences.after(today + timedelta(days=1), inc=True)
            return timezone.make_aware(datetime.datetime.combine(next_date.date(), self.start_time))

        except AttributeError:
            return None

signals.py

@receiver(post_save, sender=Training)
def calculate_next_occurrence(sender, instance, *args, **kwargs):
    if instance.next_occurrence:
        if instance.next_occurrence_celery_task_id:
            AsyncResult(str(instance.next_occurrence_celery_task_id)).revoke()

        result = calculate_next_training_occurrence.apply_async(
            args=(instance.id, ),
            eta=instance.next_occurrence,
            retry=False,
        )
        Training.objects.filter(id=instance.id).update(next_occurrence_celery_task_id=result.id)

tasks.py

@shared_task
def calculate_next_training_occurrence(training_id: int):
    try:
        training = Training.objects.get(id=training_id)
        training.next_occurrence = training.calculate_next_occurrence()
        training.save(update_fields=['next_occurrence'])

    except ObjectDoesNotExist:
        pass