People seem to have become millionaires in previous bull cycles with modest investments in BTC & ETH. If I invested in 1 BTC, is it enough to make me a millionaire by the next bull run, or are there an elite number of alt coins that could help me safely achieve what BTC & ETH once did?
I don't think 1
BTC is enough to get millionaire on the next bull run. Last ATH was 69,000$, so even on the most optmistic scenario it's unlikely Bitcoin is going to cross the 1$ million dollars barrier... Moreover, indeed, the percentage of gains with BTC must be lower than it was on previous cycles. You can't make the same profit right now early investors made on their times. Market loses traction as it becomes more solid and less volatile.
On the other hand, there isn't anything like "elite altcoins" on this industry, neither they are safe investments. The safest investment in crypto is still Bitcoin and if that is the characteristic you are looking for, you should invest your funds in BTC. All the other bets will involve a lot of risks which may not end as you intend.