What you are describing is what we used to call a “Godfather”(after the movie, yes) or “RPG” gamemode, in the SA:MP days. Basically, you take an RP server, remove the tryhard rules like Metagaming, Powergaming, RDM, VDM, etc, you create an economy and an endgame system (usually Factions and Turfs) and then you play.
It’s exactly how I made my server, and yes, you’re right, QBCore or ESX or vRP contains too much… bloat… but that doesn’t mean you can’t really use it. You can still spin up a QBCore server and have all the GTA Online Freeroam fun that you can have, while keeping an active Economy server. You just need relaxed rules and more permissions for the regular players.
Sidenote: I think we should call this type of gamemode "Economy Freeroam".
If you already know how to write code and you’re determined to make your own gamemode, I suggest you take a look at how I designed my FiveDivision gamemode. It’s not pretty or really fast, but it gets the job done and it’s really easy to connect to standalone resources. (I use it on my RPG as well, and it held 100+ people with no problems at all).
But if you haven’t made any resources before, start with QBCore and work from there. It will take a bit to clear it up for Economy Freeroam, but after that you will have a lot of really good (and a lot more of not so good) resources on the forums, both free and paid.
I also suggest you read @Jaymo 's How hackers can exploit your servers and what to do about it and Discussion: Roleplay Frameworks plus all of their replies.
Other great posts for modern resources are ofc: my Some Game Events and how to use them, @AvarianKnight 's How to use State Bags, @MrDankKetchup 's [Tutorial] How to use Routing Buckets easily :) / The correct way to instance people and @kn0pee 's [How-to] Use the profiler command to identify problematic scripts
There are other great posts on the forum for whatever else you need. You the search function with confidence.
If you’re planning on writing your own gamemode from scratch, I suggest you do as much as possible server-side (including giving weapons and spawning entities), so you can use SetRoutingBucketEntityLockdownMode - FiveM Natives @ Cfx.re Docs in Strict mode (pretty much prevented all trolling attempts on my server).
If you need anything, just ask on the forums, I’m sure there’s still people willing to help others whenever they can.