Even at $1/month for an account, 5 bots would cost $60 per year, and for most that is arguably increasing the cost too much to run a sophisticated bot net - though there certainly will be domestic and foreign bad actors wanting to infect a system for their censorship-suppression-narrative control apparatus and will be willing to spend $ billions if not $10s of billions annually - as a war-invasion-infiltration cost - but at least then they are paying into the system that their adversary controls.
This is a problem I am going to face system design wise in the next few years as I'll want to launch my solution to many problems. A core value for the network of platforms and central platform with many feature sets, is that it will not have advertising - no ads - as I believe cheap-shallow advertising is very harmful and dangerous to society. Not only do ads capture our attention that has already been, for many if not most, been compromised, it also increases the costs of goods and services - because you're funding their advertising budget when you are manipulated to purchase from them, paying for yourself to be manipulated and perpetuating that system, it also influences design decisions.
It's going to be a very challenging and ongoing cost to strike a balance, as you say, between trust and isolationism - determining what limited "Free" accounts get access to do functionality wise vs. providing a low enough cost and a high enough value to sooner than later have the masses signup, with having a large enough % of paying subscribers to cover or subsidize the cost of the free accounts holders.
I'm most torn at whether free accounts will have any ability to publicly comment, post, etc - but my current intuition is telling me that in most circumstances, no. Doing proper moderation for posts and comments from paid subscribers alone, which will include bad actors-paid shills or uncritical indoctrinated-misguided ideologues party to a mob believing and parroting propaganda-narrative talking points, that will certainly be the most important and overwhelming task from my estimation.
I think we need to rally the awake and most conscious people of the world, who arguably will also be successful in their own right, where say paying $5/month for general access is pocket change in comparison to the importance of a well-designed and desperately needed system for our security-safety and leading the charge or holding the line for peace. Likewise, I wonder then if a version of Reddit's forum, at least one method that can be enabled to limit interference, are introducing mechanisms like say $0.25 to $1/month to be part of that forum. That could then help subsidize the cost of paid moderators, who arguably would be less likely to be bad actors attempting to inject bias, etc.
My greatest concern though is that as the tyrant wannabes with global totalitarian state wet dreams start to lose their grip, as they appear to be in certain parts of the world, that they will go "full tyrant" with major false flag attacks including manufacturing consent to have censorship capabilities for not only internet but private communications; and so then an online platform is moot.