So as you’d expect, this is going to be a very “YMMV” situation…
I just started contributing to an SEP-IRA last year (my first year with profits). In my case it's entirely a tax+savings game - I'm still working full-time so I have the luxury of putting aside extra profits from my sole prop to add some additional "oomph" to my retirement funds in the near term. Because of this, I consider anything more than a couple hundred dollars saved “worth it”. I've been storing the funds intended for it in a brokerage account/HYSA and only make the contribution as I'm doing taxes for the prior year, after I can calculate my contribution limit. It could be argued it's not as efficient as investing a lump sum early or on a schedule, but it lets me manage business+personal cash flow better. If I had a business with employees other than myself I'd definitely have to re-evaluate my plan and methodology.
You mention you are investing up to $1000/month in a brokerage account - is this money you foresee a need for before retirement? If not, I could see a benefit to taking advantage of the tax savings on it now by putting some (you'll have to check how much you are allowed to contribute and evaluate what you’re comfortable with) into an SEP-IRA. It should net you the same investment options as your taxable account, save you some tax money, and give you somewhere potentially lower-cost to invest your old 401k funds. If you do think you need that money more liquid but want to keep your options open, you can still evaluate yearly whether you want contribute or not - you have until your tax filing deadline to actually make the contribution. I’d estimate that roughly, contributing to an SEP-IRA should give you savings akin to contributing to a 401k. If you do go ahead with it, there are some providers without administrative costs/fees and minimal effort to administer so you likely won't be burning time and money unnecessarily if you pause contributions at some point.
