Mikrotik AC2 with soft v7 have hardware acceleration with OpenVPN. It work great and it is big improvement with OpenVPN bandwidth -> about twice better.

But I have a problem when Internet connection is PPPOE (PPP over Etherent) but PPPOE require Ethernet with VLAN=35. So I have to first create interface VLAN=35 and then PPPOE over VLAN Interface. It's common way to connect to internet for main polish internet operator: Orange.

With such connection with soft 6.49 Open VPN with AES 128 encrypting provide 3,7-4,5 MB/s download and about 3Mb/s upload
after upgrading soft to 7.3.1 (the same 7.4) with NO changes in configuration
download is a bit lower about 3-3,5MB/s but upload is much worse 1-1,5 MB/s (remember it should be better because of hardware acceleration)

OpenVPN with Mikrotik AC2 with soft v7 with plain PPPOE (without VLAN) provides 6-7MB/s download and about 4-5MB/s upload.
Both connections (with VLAN and no VLAN) are optic-fiber 300/100Mbit.

Bandwidth for NATed and normal other operations are OK and there is no visible difference when internet is PPPOE with VLAN or without. The problem is only with OpenVPN.

Is OpenVPN required, or is Wireguard perhaps a possible alternative?

I haven't done performance testing of the OVPN since the addition of hardware acceleration in version 7.2. However, prior to the release of this version, OVPN showed very poor performance results. You can find the practical results for this router when using other VPN protocols in the topic WireGuard vs IPSec performance