1. You'll have to define "slow" - I have a 3TB table where an index only scan takes under 1ms

2. hot_standby_feedback is absolutely safe. I've got 5 hot standbys in prod with that flag enabled

3. Again, it depends on how "heavy" your update throughput is. It is definitely tough to find the right balance to configure autovacuum between "so slow that it can't keep up" and "so fast that it eats up all your I/O"