I assume by proportional relays you mean solid state? No i am not using solid state. I am going with normal relays. but with a master high load to shutdown all if a overheat problem is detected. I will be writing my own code from start to finish on this including the logic to handle sensor failures, relay always on, and alarms. The way the heat will run is like I would without being on a dimmer. something that most of us do currently anyway.
Although I am not sure why you would need high voltage dimmers? normal 120v is all you need. Unless you mean high AMP/wattage dimmers? even still my entire system with 20~ft of 12inch heat tape is about 400watt give or take. if you separate out your heat tape to smaller zones you should be able to accommodate that fairly easily. Meaning, cut the tape and have the a dimmer per every X ft and not one long strand of tape, wire in parallel.