Welcome to our forum.
While you can fit four #6 wires in a 3/4" conduit, (that's the max, by the way) I'd go the extra step and run 1" conduit too. That will make the pull easier.
What i think you would need is #8/3 wire which consists of a RED, BLACK and either a BLUE or WHITE,and a bare copper ground wire
Why 4 wire? What i think you would need is #8/3 wire which consists of a RED, BLACK and either a BLUE or WHITE,and a bare copper ground wire(teck cable)...#8 wire is good for 45 amps-in which you could use a 40 amp gfci breaker for your 30 amp load----check the name plate on spa which will have volts, fla(full load amps) which is what the tub will draw in current(amps) with all pumps, heaters and accessories on and running and that is how you would tell what size of breaker and cable sizing.I don't understand the concept of going bigger IN CASE in the future you may want to expand? If you want more buy it now...I suggest buy what you need not well maybe....