How long are your projects? I know a film editor who has edited 5 minute 4K projects -
in native 4K - on a retina 12" MacBook. Obviously took a while to export, but FCP is so well optimised, it was fine to edit with. They are currently editing a 60 minute 1080p project on a 2018 13" MBP due to the coronavirus lockdown. It's perfectly doable, as most exporting is left overnight/over lunch anyway. When you are actually editing and working on the timeline, the 13" is really capable.
The 16" will be quite comfortable for years to come. I echo the sentiment many people have said here - "future proofing", these days, is a concept that just makes people spend more than they needed to. Any big upgrades down the line are likely to be ones that you can't future-proof against today.
For example, the last time I upgraded my Mac was a 15" 2011 MBP to a 13" 2016 MBP. This wasn't because of speed - the 2011 was still more than quick enough for me at the time. However, the 2016 had USB-C; a Retina screen; Bluetooth 4 for Handoff, Continuity and Apple Watch unlock; more modern WiFi, etc. No matter how much I spent in 2011, I could not have future-proofed against this.
I think it's the same now. If you are asking whether you need an i9/32GB RAM etc, you probably don't. You would know if you did. Therefore, any money spent now is unlikely to actually extend the life of the computer, as things like USB 4, different processor architecture, better screens, better connectivity (UWB?) are more likely to come in the next decade and be worth the upgrade than a faster processor that could be mitigated by buying an i9 now.
I'd recommend buying the base 16" and enjoying it, as it should last you a long time