Update:
After considering the options, I have decided to go with Obsidian for this. After playing around with it for a bit, I realise that it will solve multiple other note-taking headaches I’ve been having too.
Sure, it’s much more of a DYI solution requiring upfront work than going for, say, a dedicated CRM app, but also infinitely more flexible.
Also, it looks like a lot of fun.
If you search “contacts” in the community plugins, you may find one that’s helpful for that. I believe there are also some CRM-related tutorials, scripts, etc. on the Obsidian Forum and elsewhere.
I use the Notes field in Contacts and have added an additional field referencing emails that are substantive.
Why did LinkedIn ruin it?
Both absolutely true. Plus when you roll your own you only have yourself to blame if it doesn’t work exactly the way you want it to, AND you can fix it yourself. 
It’s amazing to me that this topic is coming up because i’ve been working on this very thing. Re-designing a CRM type system that does not require me to buy into another platform. I’ve looked at various tools over the years, and nothing quite does what I want.
Now begins a side quest about my journey, feel to skip to the end of this block
Back in the day I used to be one of those giant-rolodex guys. I would meticulously write notes on all the calling cards in my 'dex. I had a friend who had a second rolodex that was organized by company instead of by name, so he could find people two different ways. The tactile feel of that system and seeing my scrawling on business cards made that system fun despite the shortcomings.
Additionally, I’m a big believer in the Mackay 66 Customer Profile that Harvey Mackay talked about in his book Swim with the Sharks without Being Eaten Alive. He also posted an article about it on Inc.
The problem that I’ve found is that there is no great tool to capture all this information. My current system is that I just put notes in Apple Contacts to store all of the Makay 66 type information. I also use that to keep notes about information that I learn about a person during phone calls, lunches, etc. I do use a tool billed as a CRM, Contacts+, but I don’t love it and don’t recommend it. The reason I subscribe to it is because it keeps my office Outlook contacts database in sync with my Apple contacts database.
Ideally, my system would be able to capture all the information that I want to track. It would allow me to mine it, e.g., search for people who all went to the same university. It would also keep track of conversations and meetings and remind me if I have talked to someone in more than X days. Finally, it also would allow me to keep everything in the same database rather than use two tools like notes (obsidian, notion, drafts, One Note, GoodNotes, whatever!) and contacts. That’s my dream.
End Side Quest
To weigh in with one more option that I’ve recently come across, take a look at this. There is a YouTuber who, about a month ago, posted his methodology for rolling his own CRM using Contacts, Apple Notes, and Shortcuts.
I’m not sure this ideal, but it’s given me a bunch of ideas. Maybe this could help you formulate your solution.
Also, I’d appreciate if you keep us posted on where you end up.
In the meantime, I’m going to take a look at @ryanjamurphy’s recommendation for Clay. I’ve never heard of it before.
That’s the same basic way that I do it. How do you create your fields in notes? I use all caps text set off by asterisks, and some other tricks like that. It enables some rudimentary searching, plus on my Mac I can make smart lists. (Although, since smart lists aren’t available on iOS/iPadOS, I haven’t fully embraced them.)
I too am looking for a personal CRM, so really glad to read this thread. I have tried MonicaHQ and while I’m impressed, it appears to be only web-based and as such, there’s no way that I know of to have it sync with Contacts, at least not automatically. I’m definitely looking forward to following up some of the suggestions others have made here, it’s wonderful knowing that where I thought possibly no solution exists, it turns out that many solutions exist.
Before I retired I used:
It’s very good and Free for single user.
In BusyContacts you can add an “attachment” field to a contact that links to any file you choose. So your journal of notes for the contact can just be a running text file.
Alternately you can use a url field in the contact to track any file or entry that can be identified by a url such as a journal entry in Day One.