You will want to do the following Spam Prevention items mentioned in this guide: https://invisioncommunity.com/4guides/security-and-rules/spam-prevention-r9/
Looking at your registration form, you are still using CAPTCHA2. You will want to switch to hCAPTCHA to prevent more automated spam bots.
Check that your Spam Defense is configured correctly for our services in ACP -> Members -> Spam Prevention.
Configure the Flag as Spammer option to be used by you and your administrator/moderator teams to quickly remove spam posts and ban spammers.
You will also want to rotate your Question and Answer challenges frequently and ensure that they are things which you are target audience knows but is not easily Googled. This will prevent spam human users from registering.
If you are seeing spammers from a certain country that your community does not serve, you can also block them in ACP -> Members -> Spam Prevention -> Geolocation Settings.
Finally, if you believe spammers are gaining access to accounts through means of exposed credentials from the dark web. Enabling and requiring Two Factor Authentication will help prevent that.
Outside of the items mentioned above, the next steps would be to take moderation action. Require your base member group to have 1 or more posts approved by a moderator prior to them showing up to the rest of your community without being moderated. Use the automated moderation tools so that if a post is reported x times as spam, the system will automatically hide it for your team to review.
If any spammers do get through, be sure to use the Flag as Spammer option as that will report it to our system and help your fellow administrators.
I will say that no 1 spam prevention method will be 100%. However, hopefully, with all the above, it should cut enough down that you are able to not just wake up to a bunch of spam posts that plague your community. If you deploy the moderation techniques, you will not have your community publicly plagued by spammers.
Unfortunately, in the event that a spammer has dormant account(s) on your site and they have already surpassed an acceptable amount of posts (I say acceptable as some may be borderline that your moderation team may still allow) to bypass the moderation queue, the only thing that will help are successful moderation practices by humans and staying vigilant about the future with the above.