This website has been a tremendous resource to me in my practical shooting "career" (lol) and the same for virtually everyone who has ever stepped foot into the forum. It's been 7 years since I registered my account and I probably lurked for a good 2-3 years prior to that. I would be gutted to see it disappear. There truly is no other forum (which I'm aware of) dedicated to practical/competitive shooting with even 1/10th the traffic, participants, etc. BE is a truly unique resource and if it were to be shut down, the wealth of information that would be lost is staggering.
As an internet denizen of 20+ years, and current employee in the technology/web application world, I can absolutely guarantee that charging an access fee to the general forum will ensure the forum's rapid death. The worst thing you could do is implement a change that would be rejected by the majority because most (but not all) people expect the internet to be free. It would dramatically lower site traffic which would instantly tank the value of the forum to advertisers. It would also tank the "value" of the forum to regular users, if the depth and breadth of knowledge here were to be significantly cut. Most people come here for advice/experience. If there's no one left to answer questions or discuss shooting, no one will bother posting at all.
I don't think the proposal as laid out in the OP would be successful. The Classified forum doesn't have nearly the traffic that would warrant $180/year in fees. Virtually no one would participate. Perhaps in early 2019 the Classifieds were busy enough to warrant a charge, but once the "new" moderation scheme was implemented, my perception is that utilization dropped dramatically. Only a person with access to the traffic logs can say for sure, but I know I spend less time on Enos than I used to. The Classifieds were the major draw that would encourage me to log in multiple times per day to hunt for good deals, because they would often be sold in mere minutes. Now I swing through much less frequently and see fantastic deals sit for days on end. People don't want to risk catching a ban due to subjective criteria, so they just don't list items here and use other forums instead. I know I've stopped posting items here unless they're truly USPSA specific, whereas I used to be unafraid to list any shooting-related items. And I don't think anyone could look at my posting history and argue that I've abused the classifieds or posted anything other than direct technical discussion of practical shooting, yet still, I don't use the Classifieds, ergo the entire forums, nearly as much anymore.
My humble suggestion for keeping the forums solvent:
Implement banner ads to generate reliable base income. Targeted ads from your vendors and other firearms retails would be highly successful and be worth significant CPC to advertisers. I know I would click on an ad for a good sale on bullets/guns/etc.
Tiered subscription services:
Free - Unlimited forum usage. Limit on stored PMs (50?). No post signature. 50 posts before Classified access, and thereafter limited to 1 post per month in the Classifieds. If possible with forum software, also limit to only 1 bump per week. Limit search to only 1 every 60 seconds (saves significant server resources).
Enthusiast - $5/month. Increase to 500 stored PMs. Immediate Classified access with 3 post per month limit, 1 bump per thread every 72 hours.
Supporter - $15/month. All ads hidden. For the real BE diehards who would happily pay any nominal amount to support the forums. No limit on PM storage. Special badge/icon to identify under their avatar. Dedicated forum only for Supporter level accounts. Immediate classified access, limit 10 posts per month, 1 bump per thread every 24 hours, perhaps allow editing longer than 24 hours? Would make it much easier to manage sales posts for an individual user.
Premium - $25/month, pre-paid annually. Same benefits as Supporter, plus 2 free tubs of slide glide each year and a 30 minute phone call once a year with Brian to ask a question relating to practical shooting, zen, mindset, philosophy, etc. I don't expect too many of these to sell, but it sure would be neat.
When a Classified post is closed (i.e item is sold) auto-send a PM reminding the seller that donations, even 1% of the value of the sold items, help keep the forums afloat.
Affiliate links to Amazon and other e-tailers (you might be doing this already, not sure)
This sort of arrangement preserves (maybe even enhances?) site traffic which is what keeps the forum valuable, generates both advertising and user revenue, and doesn't inhibit anyone from participating in the discussion of practical shooting. Users can stop worrying if their content to classifieds ratio is high enough to avoid being spanked by the mods.The mods themselves would have a significant reduction in their moderation responsibilities and make life easier.
I will remark that the one limit which I've seen implemented on other sites, but would urge you to avoid, are limitations on posting photos. For any pursuit relying on nuanced technical/mechanical detail (i.e. firearms), a picture is worth a thousand words. Keep the photo posting as pain-free and seamless as possible to ensure high value site content.
I know that I would absolutely sign up for the "Enthusiast" level account if it were structured similar to the above. The way to scale revenue up on the internet is keep the costs low and the conversions high. Lower fees entice far more people to spend money, resulting in far higher revenue. $5/month is fairly easy to swallow. $3/month would be even easier - perhaps $36/year annual fee if prepaid? And to put my money where my mouth is, I just purchased your book and a tub of Slideglide. (Note: I initially went to make a donation, but the donation site isn't SSL secured, so I wasn't about to enter my CC details. Made a purchase instead. Secure the donation site and I'll go make a donation as well).
For what it's worth, I did a quick calculation, and my (highly conservative) estimates show that Google AdSense would pay out ~$300/month. A higher CTR/CPC (which would likely be the case for these forums, due to how targeted/specific the audience is) would result in a dramatically higher revenue. Adsense would obviously not be the ideal choice for your advertising platform due to their anti-gun stance, but the indicative value remains.