To summarize the meeting with Weblate, we should use their Libre plan (Weblate Pricing) where 160k strings can be hosted for free, above this open source project costs are 50% off.
They were also quite flexible saying that we can chose to reduce the size of the plan later if required.
Next steps would be to create the account using the 160k free strings.
We will need more than this but we should agree a migration plan before exceeding this amount in order to best use the trial period for a larger plan.