but it's some altcoin that uses 10 or 22 word seed phrases?
Altcoins almost always copy algorithms from bitcoin so their word count should also be the same but keep in mind that if there is an altcoin wallet that generates such uncommon number of words then you can't use the common recovery tools for it because it requires a special algorithm considering 10 or 22 words don't correspond to the common entropy sizes (160 to 256 bit), or their word list and its total count could be different.
As mentioned after finding the correct seed I can go after altcoins in details.
I'm not sure you fully understood how this works.
You want to 'first find the correct seed' and then 'go after altcoins'?
It's not normal practice to reuse your Bitcoin wallet's seed phrase to create altcoin wallets, if that's what you meant.
Most likely, they have exactly
1 wallet derived from that seed phrase. Which means you have to find something like
btcrecover for various popular coins and run those all sequentially on the same seed. If you're unlucky and they left behind a wallet of a super obscure, unknown cryptocurrency, there is a good chance it will not be possible to find out which one it was.