I am going to buy $100 every month..
That's the best approach to accumulate Bitcoin but
I recommend you to buy only when you see a big dip in Bitcoin's price. The dips occur every once in a while and it will help you to accumulate more Bitcoin. The per month $100 should be more than enough to accumulate some share in Bitcoin network and over time you may be able to save at least a BTC if you continue doing that. I know that at current rates it will take a lot of time to accumulate a whole Bitcoin but I don't think that it's necessary to have a full Bitcoin. We can easily own some Bitcoin chunks and those will be very valuable if the price of Bitcoin grows.
Buying at the dip, will slow an investor from increasing his bitcoin portfolio fast because he will only buy only in a , and the worst of any investor that is using only this strategy to accumulate bitcoin is that it is impossible to time the dip and the bottom line of bitcoin,since nobody can predict the price movement of bitcoin and one might missout the dip. I believe that when bitcoin price was 25k+, some dip timers were still thinking that it will dip further but now we are experiencing 34k+, they might be regretting by now, because their fiat money that was in reserve will keep on depreciating, while bitcoin price keeps on increasing . If it is easy to time the dip, so many investors would have stash up their bitcoin portfolios and traders will always make profit, as OP said is the best way for a beginner to start with because regular DCA is easy and has little risk because you are hodling for long and at the same time increasing your bitcoin investment portfolio gradually. There is a time that if bitcoin price pumps higher, no need to DCA anymore but to hodli and come up with another strategy can you can use to accumulate.