I'm on an older version of Handbrake, but try this.

On a fresh install, create a 4096x4096 custom template and save the parameters. Now you may be able to set 1920x1080 custom frame size working within that template. I know that is normal display aspect for 1440x1080, but Handbrake has an overzealous no-upscaling "feature" that purports to save users from themselves. The Handbrake CLI has this restriction removed.

The "Bob deinterlace" filter converts 29.97i to good 59.94p with no need for intervention or pre-processing.

Yadif is one of a dozen deinterlacers that are used inside Decomb in various capacities. The idea is to catch combed frames the best way, not the same way, and it works pretty well. On fields that are interlaced but static, it "may" do nothing at all beyond rudimentary interpolation.

Setting the RF takes into account everything in the chain from source onward. Bumping your head against the quality ceiling by going below RF18 tells nothing.

Render a straight cut, no-fades sideshow at RF 20 and again at RF 5. You will see instantly that you cannot manipulate quality-mediated encoding to add bits to an already saturated stream! That's it's purpose. If one wants bits for bits sake, add more i-frames.

Like many good open-source projects, Handbrake seems to be going the way of personalities, politics and entitlements. Kind of hints it may no longer be free down the road...

Thanks, @Musicvid - I'll consider what you've proposed.

But for now, had an unintended consequence when importing the Handbraked MP4 into Vegas:

Somehow I (or Vegas?) turned the velocity way down on that imported deinterlaced, up-rez-ed, 59.94p MP4 and I rendered it out with fx and pan/crop/zoom (some of the latter very close-up) at normal MainConcept AVC/AAC 29.97p MP4 from within Vegas at a CBR of 20,000,000 bps:

---- That render looks like a really slow-mo. Too slow for the actual project, but useful in that I was surprised at the substantial improvement of the close-up/zoomed in parts even at this too-slow speed. Will probably look even better at desired/normal speed/rate. So I must be doing something right after all at this point.

I might not even need to use Handbrake as the final MP4 encoder if my current in-Vegas MainConcept MP4 settings give decent results:

This very slow-mo test is roughly half of the time length of the whole video and "weighs" 645 Mb. The site has a limit of 5 Gb per video, so I'm well within the margins even if give a bit more bps muscle.

I got nothing!

Except that...

[Opinion]

I don't think the pixel aspect ratio change from 1440x1080@1.33 to 1920x1080@1.0 is so complicated that you couldn't get excellent results in Vegas Pro to keep from having to fool with it in Handbrake for the deinterlacing work that you plan to do. Probably the trick is to find a way to deliver it from Vegas 10 to handbrake.

[/Opinion]

This is what I think Musicvid described:

Here are the results that I got:

I managed to "puff up" the bit rate, likely because of twice the number of frames (930 --->1857 frames). The bits/pixel*frame is increased also at Constant Quality = 18 (0.326 --->0.421).

@john_dennis - Thank you very much for taking the time to capture the settings for HB. And as you said and so did I in my last posting, if the Vegas encoded MP4 comes out good enough, I could skip HB.

Last, a couple of questions:

I am assuming that the to-be-uploaded MP4 final should NOT be at 59.94p, but more standard 27.97 fps or even 30fps, It would take twice as long to upload (perhaps?), and not knowing the processing engine the site uses, it could end up not being able to convert correctly and mess up the video. So...

1. True/false on that assumption -- and would I lose any quality (or is there any point in) making it 30 fps?

30 fps seems to be a Net standard. But as far as I know one should keep fps either identical to proportional (i.e. 1x or exactly 2x such as 59.94 in this case) to the original footage as possible all the way through.

My instinct says keep it at standard NTSC 29.97 for that final-for-upload MP4. Open to correction on that.
________

This video's file size is way below the 5 Gb per video limit due to it only being 8:21 min. long - however subsequent ones may be longer. So...

2. What is a good and muscled, rule-of-thumb bps for said 1920x1080p MP4 upload finals? (Otherwise put, what is too low, what is overkill and what is just right?)

I noticed that you essentially doubled your file's bps as per MediaInfo data.

I would stick with 29.97p. I didn't have a target bit rate in mind from Handbrake, only a quality expectation. The output bit rate can vary with frame rate, pixel dimensions and the amount of motion in the video, the phase of the moon and the way you hold your mouth while encoding.

Thanks. ".....  the phase of the moon and the way you hold your mouth while encoding." So if I howl at the moon and take my dentures* out as I do so, I'll get a better render. OK, got it. :)
(*TMI, but I had to cross over that Rubicon about 3 years ago. Got "monkey-mouth" lips now. So much for late-in-life romance potential now. Mercifully editing video isn't an outdoors beauty contest, I can stay in my cave - lol.)

I went back through the files that I've uploaded to youtube in 2017 and the bit rates are in the range 19 Mbps to 22 Mbps for 1920x1080-23.97p.

Warning: Don't take encoding or investment advice from me.

@john_dennis - Pardon insanely late reply. Forgot about this thread... It happens.
Anyway, thanks for response. No reply necessary. Just doin' da courteous thing.

(And, don't worry. I don't have and never have had enough money for investments. My "investments" are stuff that I can touch and feel -- and/or use, such as software and such.)