Best strategies for cost-effective solo mining

How can I cut down on electricity costs for solo Bitcoin mining? I'm looking for a setup that's super efficient so I can just forget about it for years. Size isn't a huge deal for me, and I’m not in a rush to solve blocks quickly.

There's no magic bullet here... If you buy hardware with the lowest Joule per Hash, you'll get the most hashes for the amount of energy you spend... It's just that simple. Some people might advise you to overclock or run different firmware in order to optimise your hardware even more, but you have to realise that when doing this, you might end up breaking your hardware or installing backdoored firmware sooner or later (and those optimizations are usually just a couple percent better... It's still more efficient hardware = more hashes for an amount of energy). What does make it complicated is that the most efficient miner might not be the most sturdy one... And if you pay to much for your energy, odds are pretty big you'll never ROI, no matter which miner you buy.

Yes it is quite annoying, like most things in life there isn't really a way to make free money, but mining comes close if you can get it to be efficient

It depends. If you are just interested in the principle of mining, and the "lottery" side of solo-mining, you should look at what Sidehack produces as miners: the R606-R909, Gekko Newpac, CompacF, these are small ASIC miners that don't consume a lot of power, and don't need to be monitored if they are well ventilated/cooled. They are very small hashrate miners, it's ideal for home mining, but you won't reach PH/s levels unless you accumulate thousands of them. Otherwise, recently Canaan had some great promotions on their online site, and the A1166 takes 47J per Th, which is pretty decent. But in any case it's case by case, it depends on your price paid per kw/h. I can advise you to look at Willi's Long term Group run: we rent 200TH/s for 6-7 months through a hashrate rental group, and we share the block reward if we have a block. It's a way to get a foot in solo mining without worrying about the hardware and its maintenance, or the electricity ; as it is rental hashrate. We recently hit a block in the Groupbuy#30 but as you know, it is totally random. More informations here if you are interested : https://bitcointalk.org/index.php?topic=5381245.0