This is just my opinion, but i guess you could do both. I mean, if it's for bitcoin, i think that it will be better for you to use if for investment, since bitcoin value is quite high, and it's so potential that the price might go up again and again. So i just think that it's such a great choice for investment. Meanwhile, you can find other altcoins that are more volatile and have high volume of transactions, i mean those that are easy to buy and sell. It'll be good for trading, since it's liquid. You can still trade and gain profits from your trading using those altcoins instead of bitcoin. If you can do both, why choose only one, right?
You are right, I have always said that the safest investment in the world is that of BTC, altcoins are much riskier, however many options and technologies such as Defi are emerging, under which many coins shine that have attracted a lot of community and investors.
I think that this BTC market the best option to buy alts is when the bullish trend has been confirmed, at that time it is safe to invest in altcoins, and when many call it the "Alts Season" since they can rise between 2x, 3x, 5x some up to 10x, at that time it is the safest option to invest and diversify money in crypto, of course there are other strategies, only this is the safest.