How good does a TT have to be for a good cartridge

It makes more sense to put 5k into a better tt and arm and use your existing cart. Then when the upgrade bug strikes again you can justify a better cart. Though it depends on if your phono stage is up to the task. You may want to upgrade your phono stage as well before you move up to a upper end cart.

In general the higher performance the cart the higher demands it puts on the other equipment in the chain. IMO

What I believe you will find is that a good cartridge will sound good, a fair cartridge may also sound good, and a bad cartridge sometimes will sound good on a pedestrian turntable. On a really excellent turntable, what you thought was a good cartridge may sound awful, and what truly is a good cartridge may sound much, much better. I had cartridges that I thought were good only to find out that they actually stink when mounted on a good turntable. In short, a good turntable is far more important than most people believe. At least, that is what I have noticed.

Confusing enough?

I remember going into London Audio while at University. This was 25 years ago. The gentleman in charge was very nice. He knew I was not in a position to buy anything, but he sat me down and put on an album side. At the conclusion of the album he came back in, moved the album to a different table, and I listened to the same album side again. At the conclusion he asked me which I preferred. I told him the first one was easily better. He first told me I was now hooked, and then explained that the first listen was a Linn Basic cartridge on a top of the line Linn Ittok arm, with the second being a top of the line Linn cartridge on the basic arm. He explained that the tonearm is the most important part of the LP chain. The TT was a Heybrook TT2.

Your tables are more then enough. Spend your money on the best arm you can afford. After that a good cartridge is easily upgradable in the future.

The phono Pre now becomes the second most important part of the analogue chain. Tubed is my first recommendation. If you don't want to deal with tubes, go with Tom Evans Groove. This should be at the top of any list.

The Scout is bottlenecked by its' lame JMW-9 arm and the MMF-5 has an entry level arm.

Save your money.

Since turntable systems all work through vibrations, you want only the right vibrations getting to the cartridge. If the deck and arm are adding vibrations, the cartridge will pick them up. A better cartridge will more easily pick up those vibrations, as they can't tell which vibrations are the right ones (from the record groove) and which are the wrong ones (from the deck and arm). With an inferior deck that adds vibrations, the result is degraded sound.

Following this logic, you'd want as good a deck and tonearm as possible, as garbage into the cartridge = garbage out of it. I'm not calling either of your decks garbage by any means, but I'm not sure it would be worth your while to go all out with a cartridge. It'll most likely be similar to a great set of speakers ruthlessly revealing all the flaws of an inferior source or amp.

I wouldn't spend much more than the cost of the deck and tonearm on the cartridge. I have a Pro-Ject 1Xpression with Speedbox II and acrylic platter, and run a Dynavector 10x5. While demoing the cartridge, I got to hear the shortcomings of the deck. It's a great match IMO, but the 10x5 is capable of better performance. One of these days when I have more disposable income, more vinyl, more time to listen to it, and my daughter is older than 14 months, I'll buy a better deck for the 10x5.