Ask to talk specifically about what breaks due to having the familiar's help, and see what effects the help has and how severe they are
I've been in this position. It's probably not a good approach to lean on the rules as written-- the DM probably already knows the relevant rules, and likely feels that they are fixing a problem and improving the game by adjusting them in this way. A player sets their character up to do something really well, and then they can easy-mode through that thing every time it comes up, and it seems like that thing may come up often-- it's a DM's nightmare! Or so it may seem.
My personal experience as a DM is that the sort of advantage you are describing generally seems a lot more significant than it is, and that there are few cases where it's actually going to be a problem to have a helpful familiar.
The first step in my dealing with that type of situation is to assess how much of an impact the familiar actually has, and then the second step is to think of ways to adjust the game to suit.
You've already started the first step, in that the DM has expressed to you that they think you will
- Have Advantage on most checks most of the time, and
- This persistent Advantage will make your character overpowered
With regard to (1), I think that this is largely a non-issue. Familiars have some abilities, but they are inferior to PCs in the scope and scale of things they can do-- they can't help with every problem. All of the PCs can already help each other in this way, at just about any time, and with more challenges than the familiar can address. So if helping is so powerful, it's worth asking why the PCs don't already do it all the time, and if the DM would arbitrarily limit player-to-player assistance if your group were to try it. Do you currently/will you subsequently have Advantage "all the time"?
(For the record, at my table players roll a d20 when attempting to help outside of combat, and only a 10 or better results in Advantage for the helped character; this makes helping possible and not too difficult, but prevents permanent Advantage).
As for (2), there's something to be gained from thinking about what kinds of things your character(s) is (are) doing when they are helped. Looking through the pockets of a fallen enemy is usually not something that is going to break anything in the game if you were more successful at it more often. Many other tasks you might attempt will be similar-- the benefits of success aren't massive, the consequences of failure aren't very important, and there is no upside to your characters failing some arbitrary number of attempts.
In any case, I advise you not to approach your DM with a suggestion that they are wrong and should change to be right, but instead to describe what you'd hoped to get from your familiar and see if you and the DM can think of a way to preserve that while addressing the DM's specific concerns.
Knowing what your DM is concerned will break is an important part of understanding why they've decided as they have, and can guide you to a solution that actually addresses their concern. The idea isn't to get arguments you can pick apart, but to more fully appreciate what game elements your DM wants to make certain still apply in pursuit of a fun game for everyone.
But generically speaking you can suggest limits that still allow your familiar to help you while ensuring that it's not the runaway problem your DM is worried about. It's fair to ask how often a specific issue might come up and what impacts there might be when it does.
As an easy example, if the DM is mainly concerned that you'll become too wealthy too quickly from thoroughly searching defeated enemies there are plenty of small changes that can offset the risk but still let you get something from the familiar:
- It's not difficult math to adjust how much wealth there is for you to
find and account for the familiar's help in finding it
- Difficulty of checks can be trivially increased to mitigate the
Advantage a helpful familiar grants
- The familiar can take a cut to send back to its boss in the Nine
- Maybe the help action isn't meaningful when searching the clothing of
corpses, no matter who is doing the helping, and so your familiar can
make their own ability check but cannot grant Advantage to anyone
- Perhaps the familiar is surly and won't condescend to assist you in
that way more than X times per day
- Maybe, when calculating how much "extra" wealth you'd expect to get
from having Advantage in searching for loot after a fight, it turns
out that you're only getting a handful of extra coins and the
additional wealth just isn't enough to matter much
Most such solutions will be homebrewed, but so is forbidding your familiar from helping. Familiars are supposed to be useful, and declaring that yours can never help you in this way isn't any less extreme than saying that it always does.
tl;dr: Don't try to convince your DM that they are wrong or too harsh. Try to understand what specific concerns they have, and why, and see if you can help address them while still playing your character as you had imagined.