There's no assurance in the top 10 coins, many there aren't always worth that spot they're occuring and in due time they get dethroned by worthy projects or other hyped projects as well and the circle continues in that way.
For example, last year project like SOLANA wasn't in the top 10 but currently it's occupying that spot at 7 (as of press), this doesn't mean I'm pointing it out as a worthy top 10 occupant as it could be over hyped as well but just trying to give you a perfect example and If you look at the list from few years back, you'll notice other top 10 coins by market capitalization, no longer occupy that position.
Your first advice is supported but the second isn't, when the market falls take that opportunity and get some discounted Bitcoin. That's mainly because while other altcoins aren't certain of raising back with the market, Bitcoin had always proven time without number that it does bounce back stronger after a setback (correction) and you wouldn't want to miss that opportunity.
The thing about top 10 is the fact that even when they go down in the ranks, they could still become a good profit. How? Well, they go up 50% while others go up 2x and in that situation they go down in the ranks because they didn't "go as high as others did" but still gave you a profit.
You may not know which ones will be 2x but you know that if the market is green then the current top 10 will be green too, just not as high as some others.
Doge for example is a bad one, it looks nearly guaranteed to go down eventually, but if the market is doing great, if bitcoin is 100k, if eth is 10k, if bnb is 1k+ then I am pretty sure even though doge won't go up as much, it will still be in green. That is the thing about top 10 ones, rarely they go down when the market goes up, they go down in rank but rarely ever in price, and that is why top 10 ones are liked and invested.