Interesting story, according to you, the purpose of this investment is long term, however, I'm wondering how you considered Bitcoin as your best investment? Don't get me wrong, but, the amounts being invested are quite small, considering Bitcoin's value. I believe that sometimes, investing in some other reputable altcoin, such as Ethereum, Cardano, Binance and many more, is deemed more profitable than purchasing Bitcoin.
Although @JJG already answered your questions and same would have been my said by me that for me bitcoin is the best in every perspective like prices, security, payment and guaranteed future growth whereas those coins you are taking about don't have a firm future ahead if there is any technical upgradation lag.Consider the ETH the gas is too high even after the London Hard Fork and so many coins have been burned till now but still there are lot of issues and compared to bitcoin memepool is sometimes empty and you can make transaction with low fees also.But for me there are some other reasons also and bitcoin is the only fisible option to me.
As @JJG said if you are planning to hold for long term like in my case then it's not an issue and you could easily put whatever amount you are willing to do so.The DCA have helped to accumulate more coins and as you are saying the investment is less then have a look and you will find till now i have put on more than $600 and still going on with the same and considering the prices owning a fraction of bitcoin is also worth it not full btc only.The prices have came far away and you could check price charts for it and even if it goes x5,x10, then imagine the retuns.
So for me I only prefer
BTC