So just my honest opinion here
I think you "want" this to be worth as much as possible, reading your "slight" defensiveness about low end evaluations and negative feedback. You hope, dream, even believe based on reasoning that it is worth 4 or even 5 figures. Who doesn't, most have been there - inexperienced domainers and dreamers.
Any domain can be worth $5K. even "terribledonotbuythisdomainname123xxghty.com" is worth $5k if someone wants to spend that on it. However, now we come to the real heart of domain sales, what potential is there someone will pay $Y for a domain. The above example, no-one is going to buy it for reg fee let alone $5k.
Online value tools are a tiny part of gathering info about what a domain is worth.
GD: $122; Nameworth: $14,450; Estibot: $630
All three are high end businesses and well known estimating tools. Yet all wildly different from one another. Do you believe $122, or $14K? Os somewhere in the middle? If you split it you are agreeing they are all inaccurate and so even that split figure is inaccurate as it's based on inaccurate initial values.
Type 3 or 4 random words into GD pricing and see what I mean, e.g. I just did:
getitovertherefish.com - GD thinks I can sell this for $155.
whencasinobringdog.com - GD thinks I can sell this for $521. Cmon...
There is obviously basic keyword based pricing happening there but no intelligence as to the collective of the other words.
"DoNotbook" is a negative CTA and so not just open to general businesses to buy and use, it's a distinct niche and so needs a distinct buyer. You discuss other "donot" sites doing well, but there is a massive difference which Joe Nichols already explained very well.
Also, you mention 1 similar site doing well. There are so many domains and websites that you can forget about this dream avenue that it reflects on your domain. For all you know, they could have dumped $100k in marketing!
DoNotBook - why, is something wrong? Are you showing me sites that I shouldn't use? If so, how do you make sales/revenue? Alternative sites that I should use? But then showing good sites to book with conflicts with "DoNotBook".
Honestly I can, like you and others, "force" a potential use case for this site, but such niches means your buyer pool is really tiny. That means many years with no sale, or never, and unless stupidly lucky, a low value at the end of managing the domain, listings, and renewal fees.
Alternatively, "DoNotPay" is directly a negative CTA but has positive connotations, as Joe said, people don't want to pay, so this is a positive thing. Such as "free" stuff or (as the site has) guides about ending a contract and getting refunds etc. And as such you shouldn't see this as some "similar" name you can compare yours to because without going to your site and seeing context, I have no clue what it could be. Whereas "do not pay" already has me intrigued - what can I save, taxes, utility, etc.
Namebio lists a trivial 12 results for domain sales starting with "donot" in all extensions. The highest being $2,5K, then 2 more at 4 figures and the rest 3 figures quickly going down to low 3 figures. As with appraisal tools, this is only part of the evaluation process, it doesn't mean yours wont sell. But it does literally mean comparative domains are just not selling whatsoever so yours better be exactly what someone needs.
There are probably better words one can get at reg fee, or just better for a similar price you may want. Such as "avoidbadhotels.com" or whatever. By this I mean that your domain is not really killer words that a lot of businesses really want.
I think over 2 maybe 3 years you may get 3 figures for it for someone with a hobby or simple website with info about saving money on booking things. However I'd avoid registering it for 10 years as this alone does not simply make a domain more valuable, if it did everyone would just buy a ton of names and make profit in 10 years. Age can potentially add to an already valuable name, but that's it.
Try putting it on all free marketplaces and see if you get any nibbles, use that (or lack of) to determine you future years strategy with it. You could get 4 figures for this, but I think it's unlikely - try it but not with a 10 year plan IMO.
As for developing it, perhaps you are experienced, if not let me tell you it's just not that simple to do, and even harder to do it and earn reasonable money from it.
Good luck.
