This tablet running LineageOS 18 is going to be more secure in some ways and less than others in comparison to your typical Android tablet.
Why it’s more secure
LineageOS is based off the Android Open Source Project which is very secure in general. As long as you download reputable apps from trusted sources like the F-Droid store or Aurora (Play) Store the odds of someone compromising your device are very low. Apps are heavily sandboxed in Android and have limited access to device files.
Because LineageOS is free and open source software you can be sure there aren’t any backdoors put into the software by manufacturers. The code is also being constantly audited by the Android community.
LineageOS will often support devices with software security updates beyond what the manufacturers will.
Why it’s less secure
The bootloader is not locked on devices running LineageOS.
If there an attacker has physical access to this device the tablet will be less secure since the software on the device can be modified.
A degoogled tablet running LineageOS is most likely going to be secure enough for the average person.
Unless you are a targeted individual with corporate or state secrets, or a journalist it should meet your security needs.