I have had one of those decathlon kits for years & still use the chain whip, cassette removal tool, crank/BB removal tools (got a better chain tool but the kit one still works fine). It’s done me proud really, though the rest of the kit tends to be a bit too plastic to work for long. The merlin tools kits looks to have ‘useful’ tool selection, abet more expensive for it (TBH a HT II tool are more useful than ISIS nowadays).
I totally agree with Oink but to spend £20+ on a set of allen (hex) keys, can make the process dauntingly expensive. Imagine having to buy a decent chain whip, cassette removal tool, crank/BB removal tools, chain tool which are needed for a simple job of drivetrain maintenance/replacement.
These tool kits can get you going maintenance wise, I thoroughly recommend them. Long term spend some cash, but just on the tools you need to…