All right, after a lot of thought, running test scenarios, and crunching numbers, I have a tentative plan in regard to tax optimization with my situation (self-employed, highly variable income, focused on being perpetually semi-retired rather than necessarily early retirement).
-----
The Plan:
1) Fully fund HSA every year, if eligible
2) Always contribute up to employer match for 401k, if available, but no more than that (unless know for sure that income from that job is going to consistently put me in the 25% tax bracket or higher - see #3)
3) Other than above, contribute to pre-tax accounts (tIRAs & Solo 401k) and/or purposefully realize capital losses ONLY IF income for the year would put me in the 25% tax bracket or higher, and even then, only enough to lower reported income to the top of the 15% tax bracket
4) Contribute full allowed amounts to Roth IRA's (allowed amount reduced by tIRA contributions if #3 applied, of course)
5) IF the year's income was low enough that did not need to use #3 above, use Tax Gain Harvesting for as many gains as can claim at the 0% LTCG tax rate
6) IF the year's income was low enough that did not need to use #3 above, and still below the top of the 15% bracket after #5 above, convert some Traditional IRA funds (which should also contain funds from 401k's from previous jobs) over to Roth IRA to bring reported income up to the top of the 15% bracket.
Anything left over after expenses and all of the above will go into my taxable brokerage account (well besides keeping about ~$10k in checking to prevent overdrafts) and/or paying off the HELOC if I had to use it as an emergency fund.
-----
Reasoning:
1) HSA's are one of the best deals the government is giving right now, its both pre-tax and post-tax benefits, you are statistically likely to be better off financially with one of the HDHPs that are required to get an HSA anyway, and if there's anything that's going to be an unexpected expense where you suddenly need to dip into savings, its likely to be a medical expense, so there's a high probability of having access to the funds if needed (especially since you can just hold on to your receipts for years and redeem them only when you really need the funds - I've been tracking them on a spreadsheet and storing digital copies of the receipts on Google Drive). The only problem I've had with an HSA so far is that when self-employed its very difficult to find a good custodian - most have high fees, poor fund choices, and require you to keep some portion of it in cash. I plan to convert mine to Saturna soon, which means only paying $14.95 once a year to dump my entire contribution into FSTVX (it costs $10 extra to dump it into VTSAX for some reason and FSTVX actually currently has an even lower expense ratio).
2) Employer match is free money, hard to justify passing this up
3) This is the controversial one that lead to me making this topic in the first place, because initially I felt like I should always max out tIRA and my Solo 401k, even if it meant pulling money out of my Brokerage to afford it. I realized though that if my goal is to always be in the 15% tax bracket for my reported normal income, yet I have a variable income from self-employment and thus need to have liquid assets available, there's no benefit to pre-tax retirement accounts once I'm already within that 15% bracket. That's because the pre-tax accounts are taxed as normal income on the gains, whereas taxable accounts are (currently, and if invested properly) not taxed on gains within this bracket. I used a compound interest calculator and found I'd end up the same either way, and sometimes even in favor of the taxable account (e.g. $10k for 25 years @ 7% returns but then taxed 15% on the whole amount ends up at $46,122.18, but $8.5k [$10k after initial 15% tax] for 25 years @ 7% returns and no tax on the gains ends up at $46,133.18). Not to mention that RMD's can make it hard to keep tax bracket low later on! That's why I decided it only made sense to use these accounts to get reported income per year down to the top of the 15% bracket and no lower.
4) Might as well get guaranteed tax-free gains even if the LTCG tax rate goes up later, as well as being a good place to put any Bonds or other investments where can't benefit from the 0% LTCG rate. There's a loss of versatility here compared to a taxable account but its much more liquid than pre-tax accounts since I can take the contributions back out at any time. I am only restricted from accessing the gains without penalty, and this restriction seems worth the above benefits.
5) I don't know how long that 0% LTCG tax rate is going to be around, so might as well take advantage of completely tax-free capital gains from any wiggle room I have in a low-income year. If nothing else, using Tax Gain Harvesting makes Tax Loss Harvesting later on (in a high-income year) more effective. This option is one of the strengths of a taxable account that is not available in IRAs and 401k's, ESPECIALLY for a variable-income situation like mine where harvesting lots of gains in a low-income year then harvesting losses in a high-income year can be significant.
6) This of course is the Roth Laddering technique to move money out of those pre-tax accounts. Since the funds in here should only be funds that would normally have been taxed at 25% (from #3), I'm saving 10% in tax from using the tIRA/401k in the first place (or more, if my bracket drops to the 10% or 0% [from standard deduction + exemptions] range) and after the conversion can now save tax on the gains as well, and of course reduce RMD's in late retirement and have access to more funds without early withdrawal tax before then.
-----
Conclusion:
With this setup, I should always have a decent chunk of my excess income going into fairly liquid accounts like taxable brokerage, Roth, and HSA. My expenses are in the low-end of the 15% bracket, so any income between there and the top of the 15% bracket would go to these accessible locations. Only income above that point, the 25% bracket or higher, would become tied up in pre-tax retirement accounts, and then only until I can convert it to my standard 15%-tax-rate funds via Roth Conversion. This should hopefully give me plenty of funds (assuming my income averages out to being a fair amount above expenses) for both before and after retirement age without depending on SEPP or having my plan be completely derailed by Roth conversions being outlawed, yet still save me taxes from keeping myself outside of the 25% income bracket (as much as I can get away with) and within the 0% LTCG bracket, as well as the tax benefits gained from Roth, HSA, and Tax Gain/Loss Harvesting. Of course, all this is assuming tax brackets and laws stay the same as they are now, which is unlikely, but I don't think this plan is too overly-dependent on tax laws staying static.
Does this sound crazy? Did I miss something critical?