This is key.

451 vs 406 is a similar situation as 700C vs 650B. The 451 usually comes in 25-451 or 28-451 which is the same ich size as 48-406 or 50-406. As other said, the 451 tyres are restricted. Then there is the issue of comfort and with the popularization of gravel riding, people are rediscovering that large tyres a better for comfort so @ equivalent rolling resistance, you will be more comfortable on 406.

I was in a similar situation with my Budget helios D7; it was limited by range, weight and comfort. As the OP, I was looking at verge x20 and x18 (and went down the rabbit hole researching the capreo groupset). I really wanted a verge X18 equivalent or a Dash P18 (I almost got both now).
For ~$300 at the time, I manage to convert my helios from a D7 spec to and X18T (drop bar) and then back the X18 (flat bar) and could do a X20 but it involved extra cost.

With this budget folder, a diyer can do a lot to convert to a premium or extra premium bike -the verge X18/x20 have the same frame as the budget verge and the Dahon MuSL had the same frame as the MuD7/P8. The main differences are weight related:
- mainly tyres and wheels, Aliexpress has litepro Aluminium wheel which are similar to the original Dahon (now Tern) Kinetix comp Pro. if memory serves, I saves 1.5kg fitting 2 kojak and 1 front kinetix com pro wheel.
- the stems :
*Seat post litepro are ~300g vs Std~700g
*TT stem (handlebar) are much lighter as mono tube, not telescopic).
*Saddle, is a personal thing but fizik told me if I wanted endurance, I should go for the larger saddle (I have a fizik gravel 160 saddle on grave bike and fabric scoop 155 on the 3 other bikes).
- Crankset, I fitted litepro before ratio weight saving per $ was good and I needed a set for 2X system. from that feature, I d advise to looking into Triathlon kit as they use big size which once fitted to a 20" give something similar to a regular bike.

The other big thing is the powertrain. you can find front derailleur bracket that allow the conversion from 1X to 2X. I tried different types and make and again the litepro are best IMO. As I said, because of cost, I went for X18 configuration (2x9) because I had a derailleur that was on a 7S bike but was design for 8 or 9 S only difference was the shifter. In the case of the zizzo Urbano, it has a altus 8 speed that works with 9speed (I run 1x9 11-36 and built a 2x9 11-28). All it needs is a alivio or sora flat shifter or sora drop depending of the handle bars.
If the OP needs more, a Tiagra 2x10 groupset would be perfect.
when it come to cassette choice, it depends of the terrain. I went for 9s 11-25 x 53/39 because I wanted close ratio. alternatively, I could go for 11-28 x 56-44 for extra speed (or 11-30)

Either way, 1x8 altus is limiting so for more involved ride, the OP needs more range,1x11 with wide range cassette or 2x9/10 with close ratio cassette.I would lean towards 2X for road riding and touring.

So it is really a question of budget:
- Spend $2500 on a tern X18 or X20 (2x9 or 2x10) or even a X11 (1x11)
- Spend $400-550 on upgrade for the Zizzo

Upgrading my helios gave me similar spec and the verge x18 10.25kg, similar range (missing the 9T rear cog which I would not use anyway) and If I could be bother to fit a rear kinetix comp pro wheel with a ultegra group set, I would get a X20 spec bike or lighter with carbon cranks like the Sram Force I have on the gravel bike.

X11 is a different beast as the hub 142 rear and 100 front