Bulgarian D visa:
Easy peasy for students and retirees. Otherwise, the three popular, but more difficult options are:
(a) Get a job offer in Bulgaria (with an employer who's happy to go through the immigration process) so you can get a D visa via Employment.
(b) If you're planning on learning Bulgarian to a decent standard (you need to get the relevant certificate), then that would enable you to get the D visa as a Freelancer.
If I were doing it, I would come over to Bulgaria for a long holiday (we're allowed 3 months visa-free), and I would find a month intensive Bulgarian language course for beginners (maybe near the sea in Varna or Bourgas). Then go somehere else (maybe Sofia) and do another month intensive course at intermediate level, and take the appropriate exam at the end. It's a couple of months and a couple of thousand euros. Not nothing, but not a major obstacle either.
(c) You can get the D visa as the Trade Representative Office (TRO) of a non-Bulgarian company, but this requires a couple of years of history in the overseas entity. If you already have your own business, then it's straightforward.
If not, you could set one up in the UK (or elsewhere) now, and put a bit of a freelance work through it over next 2 years. The amount of turnover is not very important, it just has to be in existence for a certain time, and be up to date on taxes and any corporate fees (in "good standing"). I'd imagine there are some lawyers with friends in the TRO office who can probably get this with less time since incorporation. UK companies have quite low incorporation and maintenance fees, so it's not a huge expense to have one for a couple of years (less than £500 maybe). But 2 years to a D visa via TRO is certainly a lot quicker than 20-odd years to a D visa via a UK state pension!
You should note that only one of you needs to jump through these extra hoops. Once one has the D visa, they can get residence. And wife/husband of resident also greatly simplifies D visa/residence process.
Serbia, the easier Balkan option:
I like Bulgaria a great deal, and very happy to recommend it. But Brexit has definitely made moving to somewhere warmer in the EU quite a bit harder for British citizens. Our neighbour is Serbia, and it has a similar culture, climate, and cost of living. It has a much easier residence regime where you go straight for residence (no long term visa required first), and you qualify by either buying a Serbian property, or incorporating a Serbian company (with business bank account). The Serbian company is the option chosen by freelancers.
Unfortunately, Ebay doesn't have much in the way of Serbian properties, so no pay-monthly options.
While Serbia is probably a viable choice in its own right, it could also be a stepping stone to Bulgaria. WIth a Serbian company (instead of a UK company) you are still on the clock towards Bulgarian D visa via TRO. But the Serbian company gets you residence there immediately. It's easy to divide your time between Serbia and Bulgaria, as when your 90 days (in 180 days) is up in Bulgaria, you just drive for a few hours back over the border.