I have used Variacs of all sizes over the years. i believe the "surge" you're talking about that trips your breaker is a direct result of the residual magnetic particles facing the "wrong way" during initial energization of the Variac iron core. Also, to make matters worse, expect a lot of third and fifth harmonic current content during start up. It;s not uncommon to see an eight fold increase in full load current during this brief time. So an 8 amp variacs' inrush current could be 50 to 60 amps easily. Same thing happens to Amateur Radio Operators when they energize their big giant 3 KVA plate transformers on their big old broadcast transmitters, so this problem is not just characteristic of variacs alone. The good news is that there are ways to fix this annoying problem.
The best method would be to incorporate a "zero crossing detector" that closes the relay at precisely zero volts. This method requires basic physical knowledge of how fast the relay you are using can close in conjunction with the main contactor, etc. The actual time the relay closes is typically measured in milliseconds and is programmed via microprocessor and added as part of the start circuit. This method is way too complicated and expensive for us regular guys who just want to plug a small variac into the wall.
Next would probably be one of those "soft start" circuits many folks talk about and use. I draw back to many of these simple circuits seems to be the power resistors used on the soft start boards have a tendency to burst into flames if left in the "full load" position too long. I've heard complaints.
I found a very reliable circuit that works very well for me. I use a small time delay relay board and adjust the operate time for about 1 second or so. I got these boards from BangGood for about $8 each (12 volt input with a 555 timer and a relay). I added a Mars relay (Available from air conditioning shop or A/C repairman.) The board may be energized with a small DC wallwart at start up and that's when the countdown begins. Here's a handy twist: Instead of using a power resistor across the relay contacts, use a run capacitor. 15 to 26uf at 240 VAC should be fine,. I grabbed one off of an old retired washing machine motor!
The beauty of using a run capacitor instead of a power resistor is that either way you chose to do it, the "random magnetic particles" in the iron core become "lined up" making large start up current inrush a thing of the past. Don't get me wrong, you still have the inrush, but it's "impedance controlled" much much smaller thus keeping inrush current to a manageable level.
One thing to remember is that in the unfortunate event that the contactor might fail (happens all the time) you won't end up with your power resistors bursting into flames, as there is no heat produced using a capacitor.
Regards, Hank