If you really are trying to focus on maximizing your chances of winning,
just pick a top tier (NSFW language), even for secondaries. Even though it seems like you've settled on PT as a main (which is a good call), I don't see much point in having Isabelle as a secondary based on actual potency against the meta. She could be a sleeper anti-meta pick, but if she were there'd probably be some kind of data to support it. Even if she were, I doubt you wouldn't get equal or better results with any top tier you happen to mesh with. I'm not sure PT really needs a secondary, but I'd suggest easy to use powerhouses like Wolf or Lucina instead. Maybe Peach or Snake if you mesh with their quirks well enough.
So, on one hand, I very much agree with the main thesis of "pick a top tier, don't self-sabotage." And I do have a strong disdain for both low-tier fetishization and people whining endlessly against top tiers and always rooting against them.
But character affinity is a real thing, and everyone is uniquely talented with different characters. Sometimes an unusual person is better with a high-tier that they just jive with than they would ever be with a top-tier, no matter how much they practice otherwise--or in rare cases, even a mid-tier. And in crazy flukes, it can even happen with the worst characters.
And in Brawl, I was the poster child of this. I played
a lot of Brawl--I went through 3 Wiis and the last one had 30k matches on it. And despite efforts, Jigglypuff remained my best character. It was just, somehow, a perfect fit for the way I think. I really thrived on spacing and airspeed, but could never make Wario work. In my quest for a higher tier character I could eventually play Snake at a functional level, but when grand finals rolled around and money was on the line, I'd have to face the facts that Puff really was my best shot.
And I made it work--I did pretty decently in Brawl, and made a decent amount of winnings relative to that era. I was able to really capitalize that no one out-of-state knew the Puff matchup, and learned how to squeeze that silver-lining to its maximum.
There's also the notion of non-linear goals. I don't romanticize the idea of being a K. Rool that places 33rd over a Lucina that places 17th. But suppose I did succeed where I had failed and actually trained up a MK that could get the same results as my Puff. (So same performance, but higher potential for future improvement) So I go through Brawl and achieve the same level of moderate success with MK instead of Puff. Would I have enjoyed it as much? Would the social dynamics have been different? Probably. I can't help but feel, with very high confidence, that the rewards
beyond just tournament winnings from that period of my life would have been less.
People remembered me easily, I made a lot of friends. There was a certain joy to being a bit more off in the wild west, figuring out unknown dynamics alone vs. grinding execution of solved problems. (I tried maining Falco for a day and wanted to shoot myself.) It was nice to have people usually cheering for me instead of my opponent. I could win every local in my city and in some ways still be considered the underdog.
Again, I think a lot of newer players fetishize this and it bothers me. All of this is silver-lining, not something to aspire towards or wish for. I tell people who want to win to practice top-tiers--that they can commit to a low-tier like I did only after they searched for other mains as long as I did, and proved that they are that one-in-a-million who drew the Ace of the worst suit.
To bring this back to earth...
Isabelle is only topical because I'm playing bizzarely well with her with zero practice (even in 3.1 she was tying my best characters), and she has very polarized matchups well-suited for a secondary. (Including good matchups against certain top tiers, better than PT or at least equal.) And if it's an equal matchup, better to not play the most popular character in the game that EVERYONE knows how to fight. 4.0 has me feeling bullish on Isabelle: Jab confirms into setup and kills, trap is reliable, rod is way better and kills, rolls are functional...
Spacies + rats + shotos are my worst characters; I struggle with vertical precision. This affects Joker too, to a lesser extent. I can make Peach and Lucina function but it's not a good fit either way. Snake is better, maybe my best top tier outside of PT, but it's nowhere near as natural as PT.
My biggest strengths are paitience + hard reads, which is why Incineroar jives and why I was able make lemonade out of Brawl/4 Ganon and Little Mac. It explains aspects of why I worked with Brawl Jiggs. (Brawl Pound could beat almost anything on a small hard read + good spacing, and I did a lot of read-based roll-to-grab.) It explains why PT works well--Ivysaur and Charizard harshly punish even modest reads, between grabs and a variety of their kill tools. It even explains my disproportionate success with Isabelle rod.
I have zero character loyalty (obviously), so if Isabelle doesn't pull her weight then bye felicia. But I really am optimistic about the character in 4.0! I think she's held back by a few bad matchups that will always dampen her tiering/solo-potential, but PT and/or Ness are great at those.
Edit: you should use
specifically for money matches. Come on, now. Very rare MU + stupid power = quick $$.
I made a decent chunk of change in Brawl with Falcon and Ganon money matches! Mostly Falcon dittos.
Plus, Mac ain't bad in doubles
with certain partners.