Betterspeedtest.sh doesn't work

theoretically, the ISP might be intercepting all outgoing DNS requests.

If one is worried about that, then one can always use DNS-over-TLS with Namecheap's servers.

No need, but an independent resolver that goes to the root servers (as deep as necessary) is one way to avoid an ISP's DNS manipulations (that said sometimes these are justified/required), I did not say it is the only way.

Well, then I need to trust Namecheap... I can not avoid a certain amount of trust, but both with the ISP's or Namecheap's DNS servers there is going to be a single point where manipulations seem possible. That said personally I am less concerned about my ISP's (partly legally required) censorship* and more about other DNS shenanigans like serving junk pages for non-existent domain names.... and their lack of support for DNSSEC (which might have changed, I have not tried their servers in a while)

now, that would be seriously annoying ...

/me runs his pi-holes in the free for life oracle cloud servers

There's literally always going to be at least some points of possible manipulation whenever you're relying on information provided by anyone other than yourself. Personally, I haven't found Namecheap to engage in any sort of DNS-shenanigans nor come across anything they'd have censored, so I am perfectly happy with them.

in fairness to me current ISP it was the ISP before that that considered that a great idea (albeit one that could be disabled in their customer center, but even then, what where they thinking...)

Yes, exactly, but with any forwarder solution that is going to be the same single point for all DNS traffic, while with a resolver, as far as I understand, that will depend on which downstream server responds to a given query, no?

And that is fine, I am not wanting to diss Namecheap in any way, I simply know too little about their DNS service to have a useful opinion.

Well, sure, if one really insists. Pfsense, for example, by default queries root-servers directly instead of using any ISP-provided DNS-servers, but I found that approach to often result in failed lookups and/or terribly slow lookups, which is why I use forwarding with DoT + local cache now.