I also think it is better to divide the investments into multiple wallets to reduce the risks.
I will start with the fact that the coins are never in the wallet, they're "on the blockchain" (said in an overly simplified way). This is very important to understand, for your coins safety.
Web wallets are
not safe. The website can change the code and steal your information even if it worked well for years. Phishing and clone/similar websites do that even easier.
Custodian services and exchanges need KYC, have ever changing rules you have to obey to and they're a magnet for hackers since that's where the money is.
Local wallets are as safe as your device. Smartphones are bad, windows systems also bad, so maybe a linux pc is reasonably ok.
So the real deal is cold storage, hardware wallets and so on, but even here you have to be overly careful on how and where you store your recovery seed and what companion/software wallets you use.
Splitting can be a good idea if you do it right. If you split between wallets on the same (virtually infected) computer you've achieve nothing.
Keeping the smaller "daily" funds online and the bulk (if it's the case) on cold storage or paper wallets can be a goo direction, if you know how to generate and store safely those paper wallets.
Acquiring a hardware wallet can be another pretty good direction.
All in all, some wallets are safer, but in case of those the weak link is the owner (you) and the possible mistakes.
Not long ago somebody thought about splitting and was mentioning even some exchanges. And as I said, those are usually not advised, especially for larger funds. But each and every solution depends greatly on the user, his amount of funds, his technical knowledge, his willingness for KYC, what crypto services he's using, what software he downloads and installs, how he uses his pc...
So I can give some ideas, but it's your decision to make.
PS. This looks to me like a "beginners" topic, not really "trading", maybe you want to move it to the correct place.