There are quite a few more telescopes planned, I think the next major one is the Nancy Grace Roman telescope, which is another infrared scope, made from an old NRO telescope. The decade survey called for another massive space telescope to look at the optical and UV spectrums, so Id expect that to be the next truly big flagship telescope.

>there must be a follow up mission planned

https://www.universetoday.com/139461/what-comes-after-james-...

The write up goes into detail on how missions are planned, and what is in line to follow JWST.

Since JWST was delayed ten years the successor will probably be delayed 20 years and launch in 2050.